I have run into a situation where I need to use <set> to group together a collection of books written for different modules. These are the books: http://seamframework.org/Seam3/Documentation
There is some introductory and non-module specific information that needs to be included before the User Guides themselves begin. It would be very useful to have <preface> as a permissible child of <set> so that this kind of information can be included. At the moment, I think I will have to create another (small) book to include this information.