Okay, I used the A+ Certification book by Mike Meyers, which includes hardware and O/S (which can be found here: Linky!
I got it from my school though, so yours might have it if you can save the money..
I also have a 'Computer Network Technology' class which is 4 periods a day, I'm enrolled in currently. (I'm in 11th grade, and going next year as well.. 2 year course). That class helps you to study for A+ (Hardware, and Software), and was quite helpful with getting me started.
By the way, there is a second version of the Michael Meyers book which is out now, which includes XP on the review for O/S studies. (Once my school gets it, I'll tell you how I feel of it.. but the first is excellent.)
On top of everything, I also took the practice tests at Pass It Now
, which had a few of the same questions.
Overall, I will give everyone taking this test some advice. It is a wonderful stepping stone, if you choose to go further, and a great certification to have. But, don't go in there without studying.. seriously.. there is no easy way around this, so really, study before you go.
The test is expensive.. but luckily my CNT class paid for it since my teacher felt I was ready. Good luck to everyone taking it.