I've read it, it is excellent. the movie "The Social Network" was based on this book. Zuckerburg doesn't like the book since it paints him in a negative light. Eduardo Saverin, Zuckerburg's former best friend and Facebook's co-founder, is a major contributor to the book. it is intended as non-fiction written in an easy-to-read narrative style--Ben Mazrich's usual style (he's written other biographies as well).
however I don't think it is *entirely* truthful. I think that there is what Zuckerburg says, what Saverin says, what the Winklevoss twins say, what Mazrich says, what Sean Parker says, and what the court documents say... and the truth is somewhere in the middle of all that.
I'm sure the book would be a bit different if Zuckerburg had cooperated with Mazrich on the writing of this book, but that is Zuckerburg's choice.