In the course of studying old and golden eras of detective fiction, I've become adept at finding and reading free ebooks, since the paper versions can be difficult to find the books I'd like to go through.
While I was experienced with reading ebooks purchased from Amazon I had been reluctant to access free ebooks and resources because the instructions on how to download and read seemed so much harder compared to reading ebooks in the Kindle app.
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Local libraries usually provide free access to ebooks via services like OverDrive and BorrowBox which you can use to borrow with your library card. Find out what ebooks the library in your area has to provide by visiting their website. If you do not have a library card, you'll be able to register to get a library card online.
Fadedpages is an online library of free ebooks created by volunteers across the World who convert the ebooks to digital format and then release them as books in the public domain. It's my top choice for ebooks in the public domain because I find their website easy to navigate. They also offer epub versions of their books that I can download and read in iBooks for my iPad.
Open Library is a part that is part of the Internet Archive and provides online access to numerous public domain and out-of-print books. Open Library is the site I use to access difficult books that aren't readily available on other websites as ebooks since the collection consists of scanned copies of books from libraries' collections, library scraps, and donations.