I actually find this quite infuriating too. I am used to creating photo books in Aperture where one can size the photo frames individually to suit the size of the photo.
In view of the fact that Aperture might not be around for much longer, I checked out the Photos app - especially the creation of photo books there. While there are plenty of layout choices, none of them works at all when one has differently sized photos from what Apple sees as the proscribed format. Well, excuse me, Apple, but like very many photographers I like to crop my shots! That means I often have long, skinny or wider-than-normal photos.
There must be a way to get the frame to fit whatever size photo I've got - it was always possible in Aperture to move each side of the photo frame individually in or out to really ensure that the frame fits around whatever dimension the photo. I know Photos isn't really comparable to Aperture but what happens once I can't use Aperture anymore?
Please Apple, before you stop us from ordering our photo books from Aperture, you must fix this - otherwise I just cannot order any photo books from you anymore.