This is trippy. Librarything is hosting an index of annotations and marginalia in the books of famous individuals’ libraries. The linked example is of marginalia transcribed from John Adams’ library.
Sooner or later — like, maybe after I die of old age, but someday, anyway — this kind of index is going to be built right off the annotations that professional and amateur scholars, book-lovers, and others make in their electronic texts. They’ll be able to both annotate into, and upload annotations directly from, their reading interfaces. Online will be indexes with marginalia and notes and comments, and students will be able to choose those commenters based on things like their academic (or other) street cred, their rankings by past readers, and so on.