It does seem to me that the 'web document' format might have some real advantages for creating 'web books' with multimedia integration because of the potential of using placeholders to embed content from web services such as YouTube, ProfProfs, (Quizmaker) etc. Can you embed these in a PDF? (as opposed to linking). I am afraid I am not very familiar with what you can do with PDF other than create clickable/touchable links. If not then the Xara web document does seem to have some real advantages. I have authored a book in Apples iBooks Author (all graphics created in Xara Designer Pro) and it is an excellent program as it does allow a multimedia approach to authoring but the final product is limited to being played in iBooks but a Xara web book would largely be platform neutral. I am aware that epub (without Apples extensions) does also offer a pretty generic format and I was kind of hoping that Xara Designer Pro X9 would have a fixed layout ePub export but it wasn't to be. Maybe the Xara web document route offers a real alternative for mulitmedia books?
Pwalder