E-books can be very expensive.

If you can somehow arrange transportation once in a while to somewhere that has a library when they have a book sale (like Uber, a bus, a cab), library book sales are usually very inexpensive. And if you buy a book for a dollar, it won't bother you if you get rid of it. (You can also donate the books you accumulate back to the library to sell again.) You can also get cheap books at Goodwill or a thrift shop. Again, you'd have to arrange for transportation, though.

There is a website booksalefinder dot com which helps you find book sales. (I hope it isn't against the rules to post that. Everyone should know this site!)