If you have an interest in reading the early books about William Blake as well as what is being written about him in our own day, you will find a tremendous resource in Google books. Many of the out of copyright Blake books have been digitized by Google. Unlike the copyright books which Google makes available, these books are published in their entirety without deletions. Furthermore they are available as text files as well as image files so that passages can be copied and saved for your own individual use or to share with others.
You may find that early works about Blake's writing offers critical information and analysis which you are less likely to find in current books. The early writers were closer to Blake's times. They were frequently giving their full attention to understanding the content of his work. Current works sometimes assume the basic mastery of Blake's content and focus on 'minute details' of literary criticism. So have a look at these out of print, out of copyright volumes which are at your fingertips.
Here is a sample passage from the introduction to The prophetic books of William Blake: Milton by William Blake, Eric Robert Dalrymple Maclagan, Archibald G Russell:
"The substance of the poem [Milton] is almost entirely autobiographical. Blake himself tells us, in one of his letters, that it is descriptive of ' the spiritual acts ' of his ' three years' slumber on the banks of ocean.' Both the characters and the action have their counterparts in the drama which had been enacted at Felpham. The disguise is often a close one: but we are told that it is a ' sublime allegory,' and ' allegory addressed to the intellectual powers, while it is altogether hidden from the corporeal understanding,' is Blake's 'definition of the most sublime poetry.' The writing was 'from immediate dictation, twelve or sometimes twenty or thirty lines at a time, without premeditation, and even against' his ' will.' ' Thus,' he writes, ' the time it has taken in writing was rendered non-existent, and an immense poem exists ... all produced without labour or study.' The purpose of the book is clearly stated on p[late]. 36, 11. 21-25 [E 137]:
'. . When Los join'd with me he took me in his fiery whirlwind:
My vegetated portion was hurried from Lambeth's shades:
He set me down in Felpham's vale and prepar'd a beautiful
Cottage for me, that in three years I might write all these visions,
To display Nature's cruel holiness: the deceits of natural religion.'"