Moving to the InDesign EPUB forum
As I understand it, epubs created through ID require cross-references (instead of buttons) and hyperlink destinations (instead of bookmarks) to link locations within documents, or within multi-document books. This works for me, although I need to be fairly cautious in designing the links. I've either used the text itself as a cross-reference to a hyperlink destination, or I have used an arrow with superimposed text that says "Go to", followed by the cross-reference formatted as "page number." This inserts the word "page" and the appropriate number. The arrow with text over it looks enough like a button to make it easy for readers to understand it.