Over the years I have found it difficult, and frustrating at times, to keep track of classroom library books. I have tried note cards, checkout "sheets", etc. Thankfully, last year I discovered Classroom Organizer. This allows teachers to create an online classroom library. Within the site students can search the library and checkout/return books. Teachers are able to create classroom settings that may include: checkout period, maximum number of books aloud, and email of overdue notices.
Classroom Organizer is especially handy when working in a 1:1 settings. This site creates ownership for the classroom library. After introducing the site and its functions, the responsibility falls mainly on the students. However, I periodically check to make sure that all students have their books checked out. At the end of the year I also create a due date for when books can no longer be checked out.