It'll be amazing next year, but not right now.
This is... uh... news to me?
You could fully shut down the Pi (which would require some external hardware or user input to wake it up again) but even then compared to usual low power electronics drawing microamps in deep sleep, the milliamps the Pi slurps up is a lot. It would probably last a few weeks, maybe months in this state, but it would also take 30+ seconds to boot every time you need to do anything.
I don't know the reasoning behind it, but it makes Wikipedia significantly less useful for this particular use case than it might otherwise be. WikiBooks might fill the gap in some areas, but it's just not got as wide coverage.
Is a fun read. It seriously considers these questions and talks about practical things, like how to salvage iron ore from scrap rather than having to resort to mining it initially.
I would like a source (1) low power, E-ink book readers with (2) an SD card slot that can handle large cards, and ideally (3) open to hacking.
This is a very small set, possibly zero. All of the android readers are ruled out because it's power overkill just to get bits off a card to flip an e-ink screen and then go back to zero power for days or weeks. There used to be some readers with SD slots but those are often limited to 32 gig. And I want (3) in case the reader doesn't handle the formats I want.
Such a machine /would/ be an offline, after civilization goes away machine. You'd want wikipedia, emergency medical handbook, plant identification books, etc etc.