Use Zathura  if you just want to read eBooks. It will save your spot on exit, and has smooth scrolling. Zoom in with = button, and zoom out with -. That’s about it, and it’s nice.
If you want more features, or you want want to have more control over fonts, contrast, layout, etc. then use KOReader .
It can do all that Zathura does but also functions as a basic book library, which is convenient. Its dual-page layout mode is good.
Use Calibre  to organize your books. even though it has too many features, it’s still quite good at what it does. Which is to say that you can load in all your eBooks of most any file format and then edit the metadata (author, publish date, title, etc.). It also supports managing the libraries of ereader devices, and keeps track of which books you have synced.
Also a lesser-known aspect of Calibre is that it includes some cli utils like ebook-convert which tends to do a better job converting PDFs to epub than most other tools I’ve tried.
Unfortunately it still seems like the best choice is to get a used kindle with a built-in backlight. Do not register it and keep it in airplane mode constantly. This prevents Amazon from accessing your device to spy on your library, delete your books, or send you advertisements. You can remove the network card on some of them if you really want (I’ve done it once). the kindles are easy to get second-hand cheaply since they are so ubiquitous due to Amazon subsidizing them to get you to buy their books. Their screen-light-battery combination is unrivaled for the price-range.
Note: Cover the logos with stickers. Tnere are one or two logos depending on the model. Most do not say “Amazon”, at least; only “Kindle”.
- Ask friends and strangers if they have eBooks to share.