15 Books.
Look here this well help you understand.
Books: 3, 6, 9, 12, 15.
Passes: 2, 4, 6, 8, 10.
Since Jackson would get 2 Passes for 3 Books, you could just add 3 onto three, and add 2 onto 2 for each 3 books you add on the total of books.
Hope this helps and have a nice day.
-R3TR0 Z3R0