That is a question you should direct to the print companies. Publishing is another 'world' apart from photography. And who says all photos are 3:2 ? Lots of cameras have different formats. Hassalblad was famous for its square format 1:1.
If you choose a 'landscape' book format (maybe 10x8") then the best you can achieve is to add your images with the option "Zoom to Fit". You will have page borders but you at least will see all your image.
"Zoom to Fit" can be set as a book preference from the main menu- Book / Book Preferences...
My screen clip shows two page templates for one image-
Adoramapix has a 9 x 12 portrait book which when open with a 2 page spread is 18 x 12 landscape, exactly the aspect ratio as the camera.