I'm sure I would have reviewed this differently if I hadn't played the games. As it is, the bulk of this book is simply a retelling of the events that happened in Assassin's Creed II and some of the memories in Assassin's Creed: Brotherhood, and having played those, reading the book after the fact proved to be a vastly inferior experience. This is why I don't usually read direct novelizations of movies or games, etc (with the exception of Star Wars: Ep. 1-6, but that's more about collecting the hardcovers more than anything). Why bother, when it's usually so much more satisfying to play the original video game, especially in the case of the action/adventure-oriented AC series? When I read video game tie-in novels, I expect more than just a rehash of events; I expect additions to the lore or the setting, even if they have to focus on other characters. Think the Mass Effect series or the Dragon Age series.
Otherwise, this book was relatively well-written. Oliver Bowden does a good job bringing the story to life with words, though the pacing felt a bit off. However, I can't fault the author much for story or plot decisions, as I'm guessing he had to stay as faithful as he could to game (another downside of direct novelizations), not to mention likely deal with a multitude of restrictions from Ubisoft.
My opinion? Skip this if you've played the game. Though, I have to say after reading this, I've gained a deeper appreciation for video-game storytelling. The industry has certainly come a long way in this regard, when the events of a game can actually be adapted into a realistic, legitimate and more than acceptable full-length novel.
