Rowan, they fit. 2 x 4 = 8. Two 4 x 6 photos = 6 x 8. And, actually, another 4x6 fits also, making 10 x 8. Still less than 8 1/2 x 11.
A few solutions:
1) Do it by making your own book pages this works.
2) You can buy pre-made book pages (Portrait Package) for various sizes of images here:
Package Deal
http://macmethod.com/ApertureBooks/Site/Package%20Deal.html
Portraits & Prints
http://www.econtechnologies.com/
I like to stay in Aperture and therefore use the pages formatted for books.
In my opinion, Aperture would be a much stronger product with this feature built in. Using book pages, you have to make a book just to print photos. Not the most elegant solution, but it works.
Clay