Another option is to only have three items in the guide section: the cover, the toc, and the document that should open first, as in Anne-Marie's example. And then you won't generate any validation errors.

It's interesting to note, though, that iBooks will open an ebook on the *first* file that contains type=”text” while it looks like Kindle wants to open the *last* file that contains type=”text”. Again, it seems like the best solution is to only have one file identified with type=”text”.