I'm not super interested in the thoughts of those I "know and trust" as I'm reading (since a circle of real friends would be too small and too unlikely to have all read the same things for this to be effective, I'm assuming this is the social-network definition of "know and trust", which is to say "follow on Twitter", so that's an even stronger no) but high-quality, extensive commentary and annotation by multiple experts would be great. Many books have some of this in the form of an introduction and some footnotes/endnotes, but I'd love the ability to turn on much higher levels of this for, say, second readings.

Unfortunately it doesn't seem like ebooks are good at handling even basic annotations—certainly no better than dead-tree books—so I'm not seeing that happening any time soon. Plus if it ever happens it'll probably be some stupid online service, which I don't want. I want it to be part of the book, like DVD commentary tracks, or at least a downloadable add-on file of some kind that sticks around as long as I want it and can be backed up.