Manhunt is the third book in the Silver Jaguar series, but luckily it doesn't really matter if you've read the first two books or not. Messner sufficiently summarizes relevant information that Manhunt is readable on its own.
The premise--the kids are junior members of a secret society that protects the world's treasures--is a bit hokey, but exactly the kind of plot that kids find intriguing. As a grown-up, the lack of parental oversight is frustrating, but it's what enables the events to occur, and a kids' adventure book wouldn't be nearly as exciting with grown-ups following the kids everywhere.
Messner has managed to cram quite a bit of historical information into the book, and her incorporation of real-life places (like Notre Dame and Shakespeare & Co.) will connect kids to the story even more.