The best known, and probably the oldest shape available on the market, is the landscape, which is ideal for every kind of picture, especially panoramic pictures. Furthermore, all photos are usually taken in landscape, therefore it is convenient to put them into a book which is also landscape shaped.
Square books can range from pocket size to larger, 12×12 size and they are quite popular nowadays. It is very likely that square books are also the most cost-friendly products of the photo book company you have chosen.
And last, but not least, portrait shaped books. These are often not offered by the photo book editor, mostly because it is only ideal for portrait pictures or (seeing the trend of not rotating mobile phones to their sides) for photos taken with mobile devices.