My last book was 11x17 landscape. Images could be full bleed horizontal, or 2-page panos, or printed with borders of various sizes. For vertical images I could have two per page if I wanted. This was several years ago, now I think I would have the book printed as one of those lay-flat books, which in my mind looks better if you do layouts of multiple photos.
Square portfolios also look good, though of course every photo will have a large border (unless you shoot a lot of square images).