I used to work in a place the required A+ cert (luckily not for my position, heh) and the majority of the people there got it after reading a book. Met a few who had taken a class and gone that route, but it's more expensive. Just depends on whichever way helps you to learn better.
Overall, it doesn't seem to be too difficult if you're a hardware-minded person, or at least a computer-oriented worker with a twinge of dedication. The test is taken in two parts I believe, both of which are something like an hour and a half to two hours long (I THINK, but could be way off). If you've done your own installations and such then you're going to know a great deal of the info, but they also delve into some slightly obscure areas (modem init strings, bios beeps and what they mean, minor current information, etc).
For the most part it seems very possible just to get a book, study for a few weeks and then go at it. As far as which book to get, I recommend going to someplace like Amazon.com, searching for A+ and then looking at which books have the best customer reviews.
