Hi, there’s a few things you can do. I’m presuming you have a kindle. Though some will help with other e-readers.. I love books & find 2nd hand/opportunity stores are good. I love terrible sci fi & get books for 20c. In my experience its bad sci fi or romanance lol. Garage sales are good too, church fairs. I’ll keep as short as I can. i hope whatever I say is helpfu.l.
ok, presuming you have a kindle. Though some are probably relevant to most e-readers.
project Gutenberg- 1000s of free & legal ebooks. They are downloaded as files *.mobi you copy onto your kindle.
Overdrive - I don’t know if this has already been mentioned, if you have a library card you can use this site. USA only.
Loan books purchased on Amazon. - you can loan your books to people. It’s in the settings on the website I think, if you want to know more, let me know. This is one thing available to people who purchase kindle books on Amazon that is kept quiet. Though it’s definitely a service that comes with purchasing kindle book. Cant loan all books, just certain titles. You have 14days to Read the loaned book.
there’s a family sharing option which is different from loaning a book to someone.
Smashwords has 1000s of free books as well as for purchase. Though I think it said over 80,000 are free.
Manybooks - new & old books. I think this one is a bit hard to navigate but has a tutorial.
DigiLibraries - all free, dl 50 per day limit.
So there are a few things you can look at. I think any e-reader may benefit from most of them.
As they are different sites with different licences they may not have some books but you’ll find it on another. Especially popular books. I think old school traditional literature is probably more common.