What app do you use to read ebooks?

Librarian here. Yea, Overdrive's near monopoly has been concerning for awhile, though libraries will en-mass drop it if they start selling user data. It's still safe from that currently.

Libby is still pretty neat.

I'm a fan of PocketBook on Android. Lots of customization, no ads. Not open source, though, and it is a little bit jank in a couple places,

I use Moon+ Reader. It's not open source but it's so snappy; page changes are instant and scrolling is flawless. It connects to my calibre server too. Android only I think.

Books - on ios

Generally Libby and Overdrive. Have said that, that's is a fascinating article. The takeover of Overdrive by KKR is very troubling...

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.

Marvin on iOS for ePubs, Goodreader for PDFs. Both solid pieces of software I've used for years.

If you're on Android, I'm a fan of Moon Reader (+ version to support dev). It'll cover pretty much any reading file including comics. That's if you already have the files.

Local library is probably the best and safest route for getting ebooks. There are libraries online that archive books as part of their mission statement. Legality is grey, but there's also archive.org and project gutenberg for books outside of copyright.

I like Librera FD. It's on F-droid if reading epubs is what you intend.