Having read it, the movie makes more sense to me; what looks "tacked on" could actually be explained pretty well.
The books (I have a version with 4 volumes) are pretty long and winded, and at some points they do start repeating themselves. They also resemble episodes of a series much more than a single epic tale like Lord of the Rings.
Anyway, if this is your first time seeing the Monkey King, you may want to watch another movie, or simply read the books. We get only a quite limited view of him here. Same with the other demons on their way to redemption.
Just kidding :), but yeah it's definitely something I want to read, but it's going to take so much time, and there are a hundred other books I'm eager to read. But I'll get around to it at some point, for sure.
I couldn't get into the books, there was just something about them that stopped me from really losing myself in them. They seemed to meander a lot. Buy i have watched nearly every representation of them on tv and movies, even the more random, or americanised stuff. But i blame Monkey Magic for that:)