I took the ASTB about 2.5 months ago, but did not use this resource. I did just take the practice test to compare to the ASTB and found that nowhereelsetogo is very correct in the 50-60% relevance.
I have a private pilot's license, so the aviation sections weren't too difficult, but if you get a FAR/AIM or a GLEIM Private Pilot's Study guide, you should get the information you'll need for those sections.
ASTB scores: 8/9/8 OAR 55
Here is what I noted:
There were a lot more word questions in the math section on the ASTB, I actually found a good study guide for the math section to be the ACT prep books as they cover more word problems than the ASTB study guides.
The mechanical and reading sections are very good practice for the ASTB: focus on the concepts and you will be fine.
For the nautical portion there are a couple guides I used such as the Basic Seamanship Manual and, if you have access, there are some Non-Resident Training Courses offered that will help, such as the "seaman" course, and even the Bluejacket's Manual. It has good information on ships/boats and nomenclature.
This isn't as specific as I'd like, but I hope it's a good stepping-off point for this resource. If you have any questions about study guides/resources I used, just ask, I'm more than happy to help as I'm sure other's on the site are as well.
Best of luck.