I was looking into ASTB stuff just to get familiar with the questions. A good one that I found on Amazon was the Trivium version, it's got two practice tests in them, and a good overview and explanation for each of the sections. I also found Peterson's "Master the Military Flight Aptitude Tests 9th Edition" online as a downloadable Pdf (not sure how legal that was). It is a huge pdf with stuff for all of the tests for all of the branches. With me only interested in ATSB, I only printed that out. It didn't go as in depth into the different sections as the Trivium version, but the additional practice test is helpful. (I bought these a year ago and they were probably not up to date because I just wanted to see what the test was like) but from the reviews they seem to still be good tools.