The best reference I know of for study is Michael Meyers "All-In-One A+ Certification" which comes with a CD quizzer and is a good reference (worth keeping even after the exam).
Also, Beachfront Quizzers were offering free study software that I thought were pretty decent - despite some typos.
I typically like Sybex and Exam Cram books but the Meyers book is outstanding so I'd only get them if you feel you need a little extra study material. Exam Crams are good to read over the last day or two before the test....
Also take a look at Cramsession.com and brainbuzz.com - their skill drills and study guides are pretty good (though they're not free anymore...unfortunately).
Good luck on the Cert...let us know how you do!