I couldn't say how many books I've purchased in the past years. Some I've used and some I haven't. As teachers, we all know some are better than others. But I have collected the ones I like and put them in my shelves and marked each book (on the inside cover) by level. I also put my studio sticker on each, so students can identify them at home as my book and not theirs.
I was broad in my levels- only making 5; A B C D and E. I did the same with my Christmas books. Now, as a part of my Sight-reading Award offered in my studio, my students can choose any book they like from the boxes BELOW their current reading level and sight-read the entire book.
Sight-reading works this way- the books should be beneath the student's current reading level, and should be easy enough for the student to just quickly read and count each one, page after page without having to go back and practice or work on any sections. This not only builds sight-reading skills, but it builds rhythm skills, and fingering skills.
The following week, I choose 3 random pieces from the book for the student to count and play. I can be a fussy or as lenient as I desire for that particular student. I can overlook a small note error, or insist on each and every dynamic and phrase. This depends on the ability of the student or if they have other learning difficulties.
When the student successfully plays the 3 pieces I picked, they receive credit for the book. If they don't, they take the book home again, and the following week I pick 3 different pieces. When they have completed 20 books, they earn the Sight Reading Trophy.