Microengravings on metal plates [1] will be more durable than electronics could ever be, and easier to read as well. No power source necessary — just a lens.
On how to operate it, it's an e-reader, you turn it on there's the manual there. There is the really difficult part of telling people "put 12V DC here, ground on the outside", but it's trivial compared to the problem of people understanding the books inside.
A diode bridge (https://en.wikipedia.org/wiki/Diode_bridge) can easily take care of polarity inversions. Voltage is a harder problem.
Since we're going through flights of fancy in these threads, another option would be to add a built-in solar panel to your hypothetical e-reader. Expose it to enough light (and you can probably assume the lighting won't be much brighter than a tropical midday sun) and it turns on.
It should still be possible to make a solar powered e-reader long-lived, but a replaceable power supply is a good idea even if your device has to last for a couple of years.
[0] : https://en.wikipedia.org/wiki/M-DISC
[1] : https://purl.pt/24107/1/iPres2013_PDF/Permanent%20Digital%20...
Same way they know now. Humans will still be humans. If my 3 year old son can figure out how my e-reader works from nothing, so can future humans.
But well, there is no demand for tech that will last for a millennium. In fact, people are pushing for degradable tech that won't stay as waste after it stops being useful instead.