I highly recommend "ASVAB for Dummies." I joined at 28 years old and was worried I had lost a lot of the high school skills/knowledge that would be on the test. To help with that I went to the library and picked up the Dummies book. It helps one find what they are weak in and just focus on that. I scored really high after using that book and that opens up all the jobs for me since I scored high enough to qualify for all of them.