Perhaps see Wikipedia on
Flashcard and
Spaced repetition. That's what this does, just using the ebook reader instead of flashcards.
And it works while you read. Read for 10 minutes (or 30 pages), then go through 25 flashcards, then continue reading for another 10 minutes. (Customizable).
This mod supports 1-sided, 2-sided, 3-sided cards; only problem is you have to create the cards as images for now. While fbink could display text directly, it doesn't work with arbitrary fonts or Japanese or ... for now. I see there is some work on that in the GitHub so maybe in future there will be a text mode too.
I'm using this with 2000 cards and the images (4000 files for question and answer side) are about ~30MB in total.