I use Kindle and Hoopla to read borrowed library books. On iPad I use MapleRead, which is my definite favorite app because it allows a lot more choice in terms of density. I do most of my reading on Android because my ereader is Android and it's just better than LCD/OLED for reading, and I use MoonReader there. I don't love it compared to MapleRead, but after a lot of experimentation it's the least bad I found on Android.
I'll also add scribd. It's a subscription service, and discoverability kind of sucks balls, but the library is OK and I usually find 20-30 books a month on there to read that aren't available from my library.