The book is a prequel to the movie though, not the series, which means that Zuko has season 3 hair from the start. Better than his season 1 bald head and pony tail look. It also means it doesn't necessarily joss the many fanfics dealing with this period.
There are a few things I didn't like though.
When Zuko is banished, he is just forced to leave the palace, then spends a few days skulking around the Fire Nation capital trying to recruit people to join his mission to find the Avatar. I thought he was banished from the Fire Nation, not just his house. And when he was looking for a ship he went to Azula? Surely not, she would laugh in his face, snap a mental picture of his misery to add to her collection and head on home. He should have made contact with Iroh, he would have been in a position to help without going through Ozai.
But all things considered, it's a very good story. Those two issues aside, there are a truly awesome bits. The beginning where the recently defeated Zuko walks through the halls of the palace hearing what people are saying but trying to appear strong until he shuts the door and collapses in tears for example. Also the scene where he removes the bandages and sees himself in the mirror for the first time. He smashes the mirror. Iroh's expression when he sees him, the crew members, and finally the picture of Zuko's own hurt, lost and confused expression. Ouch. Don't you just want to give him a hug?
All in all, it's a great story that shows us the years leading up to a story we know well. It's not really Zuko's story though. Zuko's story takes place during the Avatar series and is fascinating to watch. This neatly fills in the few remaining gaps in the story, and sets the scene for new fans while re-exciting existing viewers of the cartoon.
So yeah. Gets my seal of approval, anyway. I'm looking forward to the movie.