Animated-News points to an SF Gate article reviewing both "The Pixar Touch: The Making of a Company" and "To Infinity and Beyond!: The Story of Pixar Animation Studios".
I read "The Pixar Touch" during last week's vacation and it was quite interesting. It definitely feels like a dirty-laundry book, revealing all kinds of production troubles and more private human details (Jobs at one point thought he wouldn't have to take showers anymore), but it was still entertaining. I think though that you should take those testimonies with a grain of salt, you never know how bitter those "sources" are and how personal experiences may cloud their judgement and recollection.
I haven't read "To Infinity and Beyond" yet, but I would take these stories with a grain of salt as well, since that book was "studio-approved" (that should tell you everything...). Both books should complement each other well though.
If you want more details about Pixar's beginnings, even though the focus is on Lucasfilm afterwards, I HIGHLY recommend "Droidmaker: George Lucas and the Digital Revolution". It's filled with so many nerdy details, I loved it.
And if you are interested in more Disney behind-the-scenes details, I also recommend "Disney War".
So all in all, for an entertaining, broad, yet still in-depth coverage you should get these books: