I'm with John Mangan
I've thought about what I'd need to consult a reference book and none of the current devices have it. For starters it needs to have at least an A5 page display size with print-quality resolution. The device should be thin (5mm would be good) and, preferably, flexible. It should run for at least 24 hours between charges. This seems like a practical minimum battery life for the way such a device would be used.
My ideal form factor would be two thin, hinged covers with protective outer surfaces and a touch-sensitive A4/A5 display on the inside of each. This would let me keep the 'current' page on one display while using short cuts to flip the other between contents, index and pages selected from the contents or index. Both displays must have page forward/backward controls, and also shortcuts to the start/end of the section and chapter. Swapping between books and magazines *must* set and retrieve bookmarks.
Something that could do the above could probably handle any reading task well, specially if it supported hyper-linking in addition to the contents and indexes. Conversely, an e-reader that can't do all of the above is almost useless because it would be a pain to use.
I suppose wireless connectivity might be nice, but I don't need it, especially if its always on to annoy me with ads and run up the comms bill. USB 3 will do nicely, thank you, particularly if issues of magazines I subscribe to and books etc that I buy are automatically delivered to my home server. I'd like the ability to keep a few reference books, a magazine and a novel in the e-reader, but would expect my main library to be on the home server, not the reader, so it doesn't need more than a gig or two of non-volatile memory.
I'm happy to buy a worthwhile reader and to pay for content I need, so I *don't* want to see content I didn't ask for, especially ads.