The paperback size will fit most small paperback books. It will fit a book with a cover size of 4 1/2 inches wide by 7 1/2 inches tall and up to 1 3/4 inches thick.
The hardbound size will fit most standard hardbounds and the larger or trade size paperback. It will fit a book with a cover size of 6 1/4 inches wide by 9 1/2 inches tall and up to 2 inches thick.
The novel size will take a thick book such as a Harry Potter Hardbound. It will take a book up to 6 3/4 inches wide by 10 inches tall and up to 2 3/4 inches thick.
Any book that will fit into the hardbound size will also fit into a novel size. However, the paperback book is so small that it requires a size of it's own and will slide around in either the hardbound size or the novel size. It is not held securely in the larger size covers and it is also hard to turn the pages because of the difference in the size of the cover versus the size of the book.
