Everyone I know who has read these books is pretty enamored with them. I picked it up as a break from some of the more depressing things I've been reading. These books are fun, but still not without depth and not without their own sorrow.
The writing style is very clever and generally pretty humorous. Peter Grant is an apprentice wizard, and I appreciate that he doesn't seem to be inexplicably more powerful than anyone in history, or anything silly like that, which tends to happen in so many books featuring characters with magical abilities. He's creative and smart, but none of this feels like cheating, on the part of the author.
I'd been worried about Leslie since book one, and this is where, if you haven't read book one, you should stop reading reviews for book two. I think what was done to her is being handled well, in a believable way. The main character cares for her, but he isn't perfect, and his feelings about what has happened to her are complicated. Definitely not perfect, but I'm glad about this.
There were portions of the book early on that moved a little slow for me at first, but that doesn't tend to be the case for most people, from what I hear. I may have just been distracted or not in the right mood for reading. Once things got going, I really enjoyed it, and I finished it quickly.
As for the mystery involved, I was suspicious about one of the characters, and I was right to be, although the full story wound up being much more complicated and tragic. I was nearly brought to tears over it, but the very end of the book gives us something intriguing and hopeful to look forward to.