The description of the book shown on play.com is pretty detailed so I'll try to avoid too much repetition.
This is a beautifully illustrated book and is well structured so that you can find what you need quickly and gives spoiler warnings. It covers how to play, the main story and side quests and extras and suits all formats of the game (so not just PS3).
I bought this some weeks back before there was much (free) help on the web, although I must say, I haven't referred to it as often as I did with say Fallout 3, but if you're after to 100% complete it, then the screen capture and descriptions are a must. It also includes sections on special white and black editions of the game and "uplay" information (credits you get for completing sections of the game which you can then unlock cool things like Altair outfit, PS3 theme, bigger knife belt etc.)
It's just over 1cm thick so quite light compared to alot of strat guides, but as it's so well made and I got mine for under 10 pounds which I think it's worth it and why it got 5 stars and not 4.