I didn't mean to imply that a book should not have more than one introduction or more than one preface. Books certainly can and should, if need be.
My argument was that these should not be put into separate files and then marked as two different references with the same reference type.
The point of the guide element is to ease navigation. In situations where a book has for instance two introductions and two prefaces, I would use one of two options:
- Put the introductions into one file, and prefaces into another. Add an appropriate reference type for each. Done. This to me makes the most sense.
- Put each introduction and preface into its own file, but put one introduction directly before the other in the spine, and the same for the prefaces. Then add a guide entry for the first of each.
The first option is what I would do, but the second one is viable as well. As I said and as the spec states, the guide is there as a sort of "shortcut" to the various components of a publication. If you link to the first file of several introductions, the reader will encounter the other ones after paging through the first, provided that they are listed in the spine correctly.