I've read two Mao Zedong biographies this year. One was Mao - The Unknown Story by Jung Chang and the other was Mao Zedong by Jonathan Spence.
I liked Spence's book better because he's not so involved. Chang and her family suffered under Mao's regime and I got the feeling throughout the book that she felt the need to vilify him, taking care not to say one good word about him.
I've said before that I enjoy it when an author is passionate about his subject and I really do, but it shouldn't stand in the way of facts and objectivity. Mao certainly committed more than enough crimes and his actions can speak for themselves. There's no need for polemics. There has been criticism of the book regarding selective use of facts and evidence. I'm nowhere knowledgeable enough to judge that for myself, but it would fit with the impression I got when I read it.
The Spence biography is much shorter and much more matter of fact. It's a good first choice if you want to learn more about Mao and gives the reader the knowledge needed to better understand this part of Chinese history. I would recommend it to anyone looking for a good Mao biography. The book by Chang is not a good choice if you don't have any prior knowledge - at the very least I would recommend getting your hand on some reviews and commentaries regarding the book. That's a good idea in any case when it comes to reading non-fiction both on-line and off-line), especially when it concerns a topic you're not knowledgeable about (sorry, I got librarian-ish there for a moment).
Mao - The Unknown Story is my 45th and Mao Zedong my 46th book for the Library Challenge.