Hey folks. I guess I should tell you some stories.
You do NOT need to buy anything. The booklet is more than sufficient. I read it (the app version on my cell) in my 6h flight and scored 20. There's nothing to worry about. Just read it, you will be fine. My spouse spent 2 weekends on it (9h), she scored 19. Draw some shapes, take some notes and pls have someone take the book to quiz on random questions. You will score at least 18. Pls do a final quiz few hrs prior to the test. Do not stress too much or you will fail. Good luck.