I am interested in using the Lightroom book module, but I need to flexibility to change the aspect ratio of the book.
To my knowledge, this is not possible yet.
Any reasonable work- around for non-square books?
If you cannot use the LR templates for 8x10, 10x8, 11x13, then your only option at present is to export images and use in other software. eg. Adobe Illustrator, online book suppliers like Momento (Australia), etc.
Custom page sizes is one of the most popular requests in the official Adobe feedback forum: Lightroom: Ability to create custom book templates/sizes from scratch. Please add your vote and opinion to make it (at least a little) more likely Adobe will implement it.