Do forgive me if this review doesn't make much sense. I go all inexpressive when I try and put into words how brilliant I find Blake to be, you see. I shall keep it short and sweet - this book contains some of the greatest poetry ever written ("The Tyger", "The Little Boy Lost", "The Human Abstract", both versions of "Holy Thursday"... in fact I'm looking through the contents page of it at this moment and practically every one is truly memorable). Blake's paintings were, at times, phenomenal - the ones to accompany his poems, however, were sometimes comically childish (the tiger which stands underneath the poem of that name looks more like a child's stuffed toy). To see his best artwork, find it in the galleries in London.
I originally tried to find a nice, hardback, lavishly printed edition of this book with the prints all reproduced in the original size - then I discovered that this edition does, in fact, contain the originaly size prints - a suprise, because they're tiny. But unless you want to pay God knows how much (for they've not come on the market in my lifetime, as far as I'm aware - not that I could afford it) for one of the six surviving hand-printed-by-Blake-himself first editions, this is your next best bet. The introduction by Geoffrey Keynes is good as well, and if your interest in the man himself (Blake, not Keynes) is piqued, I recommend Peter Ackroyd's "Blake" - one of the best biographies or autobiographies I've read.
Just get this book. Read each poem in depth a million times and you still won't come close to getting every possible interpretation. Absolutely extraordinary.