For PSAT, SAT, ACT, and SAT Subject tests, my sons have used the study books available at the bookstore (Kaplan, Barron’s, etc.) as well as the official College Board and ACT study books to do practice tests and learn about strategies for certain types of problems. Some people believe that the unofficial practice tests are harder and might help students to prepare better for more difficult questions. Don’t really know if this is true, though. Generally my sons picked just one book to study from for each test and did most of their studying and practice tests in the six weeks before the test.
I have also heard that taking the SAT test multiple times might look bad, but, again, don’t really know if this is true. With my sons, I timed the SAT and ACT to be near or at the end of their precalculus class. They only took them once.