Test Prep vs Knowledge
When I did my A&P about 10 years ago the Jeppesen books were our primary text books. It was about that time that the FAA updated their AMT books. The FAA books were optional for us, but I used both.
I would say the FAA books are at least as good as the Jeppesen books if not better.
However, for taking the written test the Test Prep books or software is the way to go. Even if you have memorized all the AMT books, the test questions will still trip you up. It just the way the questions and answers are written. Some times the "wrong" answer is the correct answer for the test.
The AMT books will help you to develop a deeper understanding of the material, not just the test answers. You will need this for your Practical Test.