Everyone knows that William Blake was a great reader. What isn't generally known; what is in fact a great mystery is where he got the books or where he did the reading. I haven't succeeded in finding any information about that (tell me if there is any).
Blake went to school for part of one day; that's all the formal education he seems to have acquired. Of course he had some journeyman training. However he appeared to be the most learned person of his generation, which makes him what we call an autodidact -- self-educated.
Very likely his learning began with the Bible; that's demonstrated by the use he made of the Bible in his creative work. He had a thorough acquaintance with the Bible, but he never confined himself to a literal understanding; he didn't see it as history -- no, as poetry. The primary difference is that poetry is susceptible to various meanings, depending on the perception of the reader. Likewise the meaning of any element of the Bible is various, depending on the perception of the reader.
Among the Books of the Bible that he favored he named Ezra and Isaiah; but in MHH he mentioned Ezekiel; he had a conversation with Ezekiel (plate 13). Some of his works demonstrated a considerable acquaintance with Revelation, in the same way that John had shown an extensive acquaintance with The Old Testament.
In a Letter to Flaxman Blake wrote:
"Now my lot in the Heavens is this; Milton
lovd me in childhood & shewd me his face, Ezra
came with Isaiah the Prophet, but Shakespeare in
riper years gave me his hand; Paracelsus & Behmen
appeard to me."
If you're serious about William Blake here's your reading list:
Jacob Boehme: Aside from the Bible nothing meant more to Blake than William Law's translation of this German mystic (some would say Gnostic). The more of Boehme you read, the more Blake you will see and understand. (the English called him Behmen.)
John Milton: Blake identified strongly with Milton -- and had some marked differences with him. Paradise Lost had a great influence on him. In Plate 6 of MHH he said that Milton "was a true Poet and of the Devils party without knowing it". In Vision, writing The Book of Milton he found it necessary to call Milton back from Heaven to correct his spiritual mistakes (much like God sent his Son to save the world).
Swedenborg: Blake's parents had been attracted to this Swedish philosopher and mystic. He and his wife likely attended the Swedenborg Church in London. But he soon saw the man's deficiencies -- and lampooned him in some early works. He undoubtedly learned something from Swedenborg and lamented about him in Milton, "O Swedenborg! strongest of men, the Samson shorn by the Churches! "
Shakespeare is given by Blake as one of his significant literary relationships. I haven't found that in reading Blake. David Whitmarsh apparently has a lot of say on that score.
Paracelsus (1493-1541): To learn how this man affected Blake you might best consult Milton Percival's Circle of Destiny (probably the best introduction to Blake). He has a chapter on Alchemical Symbolism, and reading this will help you understand how and why the furnaces come up so often in the major prophecies.
Although Thomas Taylor was one year younger than Blake, his translations of Plato and the Neo-Platonists led our poet's interests emphatically in that direction. Thereafter the Greek and Roman myths loomed large in Blake's poetry and pictures.
Homer was a major source for Blake's works. Although he expressed some contempt for Homer, he drew heavily on Homer's stories, using Ovid more often than Homer himself. To get some understanding of how Blake used Homer take a look at my file on myths. The Sea of Time and Space is directly about the Odyssey, a pictorial description in fact (of course it's about a lot of other things as well).
Hermes Trismegistus was a special interest of Blake's as well as a dozen similar arcane and esoteric works too numerous to discuss in this post. But perhaps there will be more to come.