As somebody who has played the original Assassin's Creed game 5+ times in my life with several years between playthroughs, I went in skeptical as to how good this book was going to be. The book has a short intro before launching into the game, where it almost exactly mirrors it (even dialogue is word-for-word).
For me, I am very familiar with the game so while it was cool getting a refresher, it didn't do much for me. The entire book is about 450 pages, and the game ending happens at around the 250 page mark. After this, the story gets really good.
Altair's backstory and future is really fleshed out in the second half of this novel. I went back and read the first 20 pages or so once the story started getting more personal, and I would recommend you do the same as things started clicking better.
If you have played the videogame, one of the main aspects the original story lacks is character depth. Altair is cool, but the user doesn't really connect with him because we don't know much about him other than he's a master assassin who screwed up, redeemed himself and saved the order. Despite having played the original game as many times as I have, this was the first time I actually made a connection with the character and started caring about him.
This book is definitely deserving of a re-read many years down the road.