The best book is Rat Salad. Very well written and informative not only about the music, but about the social environment in which was created.
The only criticism is that it is maybe a bit too detailed regarding the individual song construction at times.
Having said that it at least feels like a properly written book.
Don't really to want to be too controversial, but Iron Man is very poorly written. It's interesting in parts (i.e. he has obviously had an interesting life), but there is not really any new insight into what it was like being in the band, the writing process, how he felt, etc. Ozzy's is much better and genuinely hilarious in parts. The part where him and Geezer bump into Robert Plant in Birmingham particularly.