When I had a subscription a few years ago, I made about 6 copies of each book and put them in manilla envelopes along with the printable worksheets and assessment pieces. I labeled the envelopes and kept them in plastic tubs, file cabinets, or magazine holders (depending on the year and how I was organizing at the time).

The outside of the envelope contained the level, title and word count. If you have accelerated reader, you can write a teacher-made quiz to go along with the book if you have the word count. The important thing is to find a system that works for you and your students. Then stick with it. Good luck!