TL;DR
I was disappointed by this novel, not only as a fan but as an "avid" reader as well.

For people who aren't fans of the game:
You aren't missing out by not reading this novel. As you can probably guess, this novel isn't written for you at all. But if you're really persistent, here's my review for those who haven't played Assassin's Creed III or any Assassin's Creed games for that matter. I'll keep it short.

I personally do not like Bowden's narrative style. It's rather dry and he downplays a lot of "should have been" big moments in the book. It lacks tension and conflict, which is rather concerning because that time period (American revolution) was full of it. There was so much that could have been done with Haytham's character and the people around him. It was just so boring. It was lifeless. It was just... There.

For the fans:

Spoiler tag for spoilers and some profanity, sorry, I have a lot of feels.

Even though I was largely disappointed by this, I did enjoy the parts that were not included in the game such as Jenny's rescue, the death of Holden, Charles Lee's "jealous" side (Charles was so jealous of Ziio, I don't care what you say, it's cannon, I ship it). As soon as I came across the opera scene, though, I got all pumped up. Kinda like, "AWEEEH YEAH SHIT'S GOING TO GO DOWN." But it didn't read as well as playing it did. I took that as a sign and in the end, I was right.

Now I'm being really picky. There were some things that were canon in game that were contradicted by this book. I'm not sure if it's canon that Haytham never knew of Ziio's death until Connor did but the his reaction to her death looked so genuine, I almost refuse to believe that he already knew.

As for the writing. It was lacking. It was so lacking. Haytham, who is quite possibly the most charming and charismatic man in the whole game (Connor is a different kind of charm and charisma) seemed dull, drab, and boring. He was like a whole different character. Oh well.