Choose the shape of your book based on the content you plan to put in it. The choices are:

Square  9×9, or 10×10 – when you have a images of various shapes a square book always works.  Smaller books – 5×5 or 8×8 are also available.

Portrait – 9×12 – suitable for weddings and memoirs with a lot of text. Miniature versions keep the same aspect ratio but are 5×6.6

Landscape – 10×8 – if most of your images are horizontal this format is best.