The comparative perspective he had - knowing a great deal about the literature, religion, society, poetry, and mythology of most of western Eurasia at the time of Zoroaster's probable existence - allowed him to fill in the blanks left by other translators, so these translations are a little different to older ones. The book is fantastic; each of the hymns is broken down into stanzas, with each hymn and each stanza having a commentary. It is clear from the translations, as well as the commentary and introduction, that Zoroaster was a real man who lived a real life and composed most of the hymns himself, and not a mythic personage. Here is some of what can be gleaned about his life and works, and his impact.
Zoroaster's name in Avestan, his native language, was Zarathushtra*, and it means 'old camel man'. His surname was Spitaama, which was the name of his clan. The word for 'camel' in Indo-Iranian languages, including Sanskrit, Avestan, and Persian, is not of Indo-European origin; it probably comes from a language spoken in the cities of the Bactria-Margiana Archaeological Complex (BMAC), a non-literate, urban, Bronze Age civilization of Central Asia. As the BMAC people left no written data, no one knows what language they spoke, nor what language family it came from - but it seems reasonable that many lexical innovations in the Indo-Iranian languages came from whatever language the BMAC people spoke, as they are found in Indo-Iranian rather than merely Indo-Aryan or Iranian languages**. *Hustra, 'camel', is one of the reconstructed terms with reflexes found commonly in Indo-Iranian languages, and it is part of the name of Zarathushtra Spitaama, prophet of the religion of Darius and Freddie Mercury.
He likely existed around the sixth or seventh centuries BCE in what is now western Central Asia - Afghanistan, Turkmenistan, Tajikistan, Pakistan - and he spoke Avestan, a language that is peculiar for several reasons. The dating of his life has been quite difficult to work out; he was illiterate, and so were all of the other people in his immediate area. It was once thought that he could have lived much earlier - around the late Bronze Age, c. 1200 BCE - but there is no reason to believe that this was so, and the other evidence points to a later date.
The Old Camel Man was a radical. He grew up in a pastoralist's world, with no state presiding over events. The problem with this is that cattle are easily rustled; they are moveable property (in fact, the Proto-Indo-European word for 'cattle' also seems to mean 'moveable property'), and cows literally move themselves about on surprisingly fast legs. Penning them up isn't always possible. Without a state or some other force preventing people from rustling cattle, when they are the primary source of sustenance and wealth it is practically an inevitability. Worse - cattle were the victims of slaughter and sacrifice among Zarathushtra's people. Yasna 29, one of Zarathushtra's hymns, is devoted to the Cow's Soul, a frequently occurring figure in the hymns. Zarathushtra questions the wisdom of sacrificing, slaughtering, and stealing cattle, on the basis that the cow suffers, and that this is wrong. The cow's soul itself asks why it has to suffer and be subject to such an ignominious existence - why can't the herdsman and the cow co-exist peacefully?
To You the cow's soul complains: 'For whom did Ye shape me? Who made me? Fury and force, cruelty, violence, and aggression hold me bound. I have no pastor but You; so show Yourselves in good pasturing'.
Zarathushtra, seemingly one of the earliest animal rights activists, grew up in an essentially Vedic society. The religion of the Rig Veda, the earliest 'Hindu' text (so-called), was the religion of the ancient Indo-Iranians. We may infer this from archaeological evidence, but also from Zarathushtra's own words. He rails against the daeevas, the Vedic deva, which he sees as demons rather than the gods the Vedic religion saw them to be. He is rebelling against the Vedic ritual he encountered: an orally-transmitted religion composed of hymns intoned by priests in a pastoral society of charioteers and horse-sacrificers. He refuses to sacrifice the cow, he refuses to steal or allow the cow to be stolen, he rebels against the notion of the demons being gods. He decides that gods are not spirits and they are not innumerable; they are finite in number, and instead of using the word daeeva to refer to them, he calls them ahura, cognate with Vedic asura, another word for deities and spirits.
Chief among these ahura is Ahura Mazdaa, which West translates as Mindful Lord. He is accompanied by the personifications of mental forces: 'right' (Asha), 'good thought' (Vohu manah), 'piety' (Aarmati), etc. Opposed to these good forces is 'wrong', and the followers of Wrong or Angra Mainyu make up the majority of mankind, those who make the cow's soul suffer and act in defiance of good thought.
Such were the very basics of the religious views of Zarathushtra Spitaama. It wasn't quite monotheism in the sense we know it today - in fact, monotheism hadn't been invented anywhere in the world at this point, as far as I'm aware; certainly the people of Canaan were polytheists at this time. There is a lot more to Zoroastrianism, both in its current form and in its ancient precepts, and I'd advise anyone interested in the diversity of religious belief or the ancestry of modern religions to read West's book or something comparable.
Zarathushtra's cult was a local one in his lifetime. He composed his hymns orally, in the same way the Rig Veda was composed (neither the Avesta, the canonical scripture of Zoroastrianism, nor the Rig Veda, was committed to writing until the early/mid first millennium CE, a thousand or more years after Zarathushtra's death), and he transmitted his works to others in the same way the brahmins of the Vedas transmitted theirs: by asking of their students not understanding of the poems but rather rigid fidelity to their sounds and syllables.
This had a curious effect: when the Avesta was finally written down in the fourth/fifth centuries CE, it was a perfectly preserved fossil of the Indo-Iranian languages, with every phoneme seemingly recorded with absolute clarity and loyalty to the very phonemes of Zarathushtra himself. Try this on for size as an amazing fact: there is a human language known to linguists only because Persian priests chanted it with perfection for over a millennium before ever writing it down.
And when they did write it down, they weren't content to inscribe it with the inaccurate script they used in daily life - which at that time, c. 5th century CE, was the Pahlavi script, an abjad (consonantal alphabet, incapable of recording vowels). No, they invented an entirely new script for the language, called, unsurprisingly, the Avestan script. This 53-letter script is a true alphabet, and it was invented entirely to record Avestan for religious purposes. It shows mild Greek influence; possibly the idea of a true alphabet was the result of knowledge of and familiarity with the Greek alphabet, and some of the letters are shaped slightly like Greek letters. But really the Avestan script was a fifth century International Phonetic Alphabet, designed as an artificial alphabet to perfectly record the sounds of an ancient language.
Zarathushtra initially had a small band of supporters. One of the reasons we can insist on such a late date for his life, in the middle of the first millennium BCE, is that the founder of the Persian (Achaemenid) empire, Cyrus (Kurush in Old Persian) 'the Great' doesn't appear to have been a Zoroastrian. His descendants were, however, meaning that at some point Zarathushtra's religion became the state religion of the most powerful empire in the world. The Persian empire under Cyrus, Cambyses, and Darius was the largest empire in the world up to that point, and it held the lion's share of the population of earth - in fact, it may have been the largest empire in history in terms of the proportion of the living human population of the universe under its yoke.
It's important to remember that the Zoroastrian religion changed considerably. Zarathushtra's emphasis on the cow's soul appears to have become unimportant, as the sacrifice of cattle continued throughout the Persian empire at every point in its history, likely a hangover of the Babylonian syncretic religion that held sway over the territory west of Iran (the neo-Babylonian empire) before its conquest by Cyrus, in which cattle sacrifice was held in high regard. There was change to the doctrine in other aspects as well, and a series of other prophets developed in the Zoroastrian fold - among them Mazdak, an early first millennium CE proto-socialist and politico-religious reformer whose name was kept as a bogeyman in Islamic Persia for centuries afterward.
Despite the changes in doctrine, it is fair to say that Zarathushtra, an illiterate Central Asian herder with a penchant for bovine rights, created the religion that endured as the doctrine of earth's most powerful empire for almost a thousand years. Under the Achaemenids, the Arsacids (Parthians), and Sassanids, it grew to become the primary religion of the area we now call Greater Persia, encompassing modern-day Iran, Afghanistan, Tajikistan, and elsewhere, and its founder, under the name Zoroaster, was esteemed by Greeks, Romans, Egyptians, and others, for as long as an empire emanated from Persia.
Among the admirers were the peoples of Canaan, people who had endured the deliberate destruction of their kingdoms and the equally deliberate forced migrations of their populations into Mesopotamia ever since the Assyrian invasions in the early first millennium BCE. These folk, espousers of a religion that became Judaism, were released from Babylon and Assyria by the Persians - and as a result, Cyrus gets a good write-up in the Hebrew Bible. Not only this, but the ideas of Zoroastrianism had a very clear and obvious impact on Judaism and, thence, Christianity and Islam. This influence is shown not only in the ideas of the Bible, but even its language and tropes. Canaanites, imprisoned and used as slaves in Babylon for several decades, returned home to their country speaking Aramaic, the language of a Syrian desert tribe, and practicing a religion they had created through the amalgamation of the Zoroastrianism they encountered among the Persians with their pre-Babylonian traditions. I find this quite a stunning idea, although it shouldn't be.
It would be hard to exaggerate the depth of Zoroastrianism's fall. Within a century or two of the inscribing of the Avesta in the wonderful Persian invention of the Avestan script, Sassanid Persia had been attacked and overrun by Arabs, a group who had been on the fringes of history until the first millennium CE. For reasons unknown***, they expanded and attacked all of their neighbours, taking Persia relatively soon after the initial expansion, c. 651 CE, despite considerable and heroic Persian resistance. In the wake of these Arab invasions, a new religion was introduced with an Arabic name (Islaam, meaning 'submission') and an Arabic scripture, the Qur'aan.
Zoroastrians found themselves taxed heavily by the new invaders. State support for the words of Zarathushtra Spitaama evaporated, and the economic and, probably, spiritual benefits of joining the Muslim majority would have seemed overwhelming for the Persian population. This was a slow process; 10th century Persian literary works like the ridiculous and fantastic epic poem Vis and Ramin attest to the respect in which Sassanid times and Zoroastrian religion were held even at that late period. But it was almost inevitable that Islam would win out in the end, and although there are still Zoroastrians in Greater Persia, the majority of the population of Iran is Muslim.
The greater part of the world's Zoroastrians today, about 100,000 of them, live in Gujarat, India, where they are known as the Parsis, which means 'Persians'. They still read and write Avestan for religious purposes, but ordinarily they speak Gujarati or English. The community is fairly closed to outsiders and is shrinking, although Parsis are active in society and especially the media. Freddie Mercury, a British-Gujarati Parsi born in Zanzibar, is probably the only Zoroastrian most people in Europe or America could name. From the thousand-year state religion of the world's most powerful empire, Zoroastrianism has become the religion of a small, closed, international community living far outside any of the known territories of Zarathushtra himself.
* I'm not going to bother putting in diacritics for Avestan words. They are written considerably differently to the way I'm writing them here, but this is simply an intro, and I'm not sure a pile of diacritics will help rouse interest in Zoroastrianism.
** Indo-Aryan is a sub-set of Indo-Iranian. Since the new vocabulary is found throughout all branches of Indo-Iranian, the loans have to be dated to the time before Indo-Aryan diverged from Iranian.
*** That's right: unknown. No record of Islam or Muhammad is found until some time later, as Tom Holland has controversially shown.