When I was studying for Linux+, Sybex was advertising their book on CompTIA's site. So that's what I bought. Though I can't really recommend the book for various reasons, the tests on the enclosed CD are fantastic. Had used the ExamCram series before, and their sample tests all require Windows. I really couldn't see myself booting Windows up to study for my Linux test.
Sybex's tests are all web-HTML based so they work on any OS. There's a quiz for each chapter and two full sample tests on their CD.