Pop up footnotes will be great. The publisher/author will have to create them correctly though. I never liked the link that took you to the footnotes page, then you had to return to where you were reading. Popup is much better. The last few books I read with footnotes/endnotes placed them at the end of each chapter, but they didn't format them as footnotes/endnotes so they were treated as the last part of the chapters. No way to link to them. Lazy publisher I guess.

With printed books, I always much preferred footnotes at the bottom of the page which referenced them over endnotes which are all located at the end of each chapter/section or at the end of the book. In graduate school I hated having to hold my place in a book and flip to the back to read the endnotes. So when I wrote my thesis I demanded the ability to use footnotes, which was a royal PITA to do in the 1980s with MS Word. The Thesis Style Approver, or whatever nonsense title she had, basically a glorified librarian at the university library, approved my formatting. I printed the thesis on the official paper and submitted it. I was very proud of the look it had. But then the Thesis Style Approver changed her frigging mind and rejected it. I had to reformat the thesis and use endnotes instead. Arrrrgh!!! What a waste of time. "What a byotch!" Those were the words of my thesis committee chairwoman, not mine.