Greg Scallon on Online Printing and Books
Subject:   Creating your own book themes
Date:   2007-03-23 09:19:55
From:   johnbeardy
To answer one of brittonfraley's questions. Why not try making your own book size? Find Aperture in Applications and select Show Package Contents, then go into Contents/Resources/Book Themes and you'll find the various settings. You should be able to copy and edit these.

However, this is not documented, and while I've done similar things elsewhere in Aperture, I have not tested this suggestion. I would feel happier if I was saving the new thems into the Library/App Support area. So take proper backups and ask in the forums before proceeding.