The case of the very skinny book!!

I was shelving a cart of items...nothing wacko about that! I was putting a book on the shelf when another book shot off the shelf at me. I caught it mid-air. My mind registered the fact that this book felt awfully skinny. When I opened the book, it was completely empty of pages...it was just the empty cover. Now that's wacko!! Many theories have been kicked around as to this books demise...I think it's just something librarians like to do!!