The Handheld Spiral is recommended for mini e-books.
The 250 square and 125 square are a very popular size for sidebar advertising and affiliates.
Get a closer look at the styles here:
Style 1 Ring Binder Standing
Style 2 Paperback Book Standing
Style 3 Clipboard
Style 4 Magazine Lying Down
Style 5 Hand Held Top Spiral
Style 6 Hardcover Standing