I'd say go onto a ratings site and see what books have received positive reviews. I went on Amazon and this McGraw-Hill GED book (http://www.amazon.com/McGraw-Hills-GED-Complete-Reliable-Program/dp/0071381791/ref=sr_1_2?ie=UTF8&s=books&qid=1265924681&sr=8-2) got good reviews. You'll probably also want to visit the Official GED Testing site (2nd link) to make sure you know what to expect and to see if there have been any recent updates to the version/year of the test you will take. There are some sample test questions on the GED Testing site as well. Finally for more relevant and inexpensive test prep, I'd visit your local library to see if they have any current GED books (maybe even some of the ones recommended on Amazon.) Even if the books you find at the library are a bit older, often the types of questions asked on the test have not changed much and you can still practice with older versions.
I also have used the McGraw-Hill GED book. All I can say is study,study,study.It's not an easy test.Most people think that it is. This test is not an A or B grade.You have to score high enough to pass.So like I said STUDY STUDY STUDY! Good Luck! I'm sure that you will do great!