I bought the Airman knowledge testing supplement for sport pilot, recreational pilot, remote pilot, and private pilot.
Have not taken the test, also free app on line that you can download on your phone. This app covers 84 questions on regulations. 92 questions on National Airspace System, 67 questions on weather. 28 questions on Loading and Performance, 70 questions on Operations. The app is called Prepware Remote Pilot, I like it because if you answer incorrectly you can mark the incorrect answer. And, do more research, like the charts, for me is very difficult to understand. The Airmen knowledge book is around $7.00 dollars, and the app is free. Any way I can study, anywhere, don't have to be sitting in front of computer all the time. Oh, the app does give you the correct answer, and explains the charts. I still have a difficult time, may be a while before I take the test.