And it's good. Oooh, it's very good. In all seriousness, I'm more excited about this book than any other.
For those of you who don't know this already, Books 3 and 4 of the series were originally one big book (full explanation here) and so Book 3 kind of ended on a big cliffhanger. So you'll want to read it just for that. But there's more. This book also solves one of the great puzzles in the series...and gives more information on an even greater one. Some mysteries are solved....and some mysteries get even deeper.
I'll say no more. You'll have to read the book to find out!