Read J Randy Tarraborrelli's 'Michael Jackson: The Magic and the Madness.' It looks beyond all the media rumours and has been compiled from over 30 years worth of interviews and research. It's unbiased and accurate, and it's very in depth therefore gives you the greatest understanding of MJ you can get from a book. It is the best book written about him I've read, and I've read quite a few.
As the people above said, do NOT but Unmasked by Ian Halperin. I believe it was only written with the sole intention to rake in sales, really. It's full of errors and all the rumours the tabloids have been publishing, e.g. Michael Jackson had skin cancer. That was a total lie yet it made it's way into this supposedly accurate, factual, in depth book. The author expresses his OWN opinions about MJ throughout and twists the facts- do not buy.
I've yet to read Moonwalk, suprisingly my mum has an original copy which she says she's willing to lend me. I've got pretty high expectations of it as it is an autobiography after all. I expect it's really good.
'Michael jackson: Life of a Legend' and 'Michael Jackson- Legend, Hero, Icon: A tribute to the King Of Pop' are apparently good tribute books. The pictures are great, also.
My friend's read 'Dancing the Dream' by MJ himself and she said it's full of beautifully written poems and showcases his talent in the arts, writing, drawing, painting, etc., other than his musical talent.