Youth fiction by Jenny Carroll/Meg Cabot.
This is the fourth book in the 1-800-WHERE-R-U series.
After I read Safe House, which I thought was book two, I found out it was book three (GRRR) and had to go back and read Code Name:Cassandra.
I hate it when I read a series out of order, but these books are actually good enough to stand alone.
Anyways, this was a great series.
So now I'll go back to grown-up fiction, I guess.