Something that would be useful: if I could flip through the bibliography of a book and see if it cites papers I recognize, and the bibliography indicated where in the book that paper was cited. That way I could tell where the book uses work that I'm already familiar with. (If I recall correctly, Graham, Knuth, and Patashnik do such a thing in their book Concrete Mathematics -- each entry in their bibliography indicates the numbers of those pages on which the cited work is referred to.)
This would be more useful for books than papers, because:
1. books are longer;
2. I'm more likely to be reading a book in dead-tree form. I usually read papers on-screen (since most of the papers I read are downloaded, and I don't print them out because I don't like drowning in paper), and most of the time the text is searchable.