Ledgable reads the book files you own: EPUB, PDF, MOBI, FB2 and CBZ. Where those files come from is up to you. Here is the honest map, including what Amazon does and does not give you.

With Amazon

Kindle books are sold in a locked format that only Amazon’s own apps open. They will not open in Ledgable, and this page will not tell you how to unlock them. If you like buying from Amazon, keep reading Kindle books in the Kindle app; the same is true of Audible audiobooks. Nothing you buy there can move here.

Without Amazon

Everything below sells or gives you a plain file that is yours:

• Standard Ebooks — public-domain classics, carefully typeset, free. Its OPDS catalog is open to members of its Patrons Circle.
• Project Gutenberg — seventy-thousand free books. It also speaks OPDS: put https://www.gutenberg.org/ebooks/search.opds/ in Settings › Books and the whole library is browsable from your shelf.
• Publishers who sell without locks — Tor, Baen, Verso, and many small presses sell EPUBs straight from their own shops.
• Humble Bundle and StoryBundle — bundles of unlocked EPUBs and PDFs, often for a charity.
• Smashwords, Leanpub, itch.io, and authors selling directly — independent writers, unlocked files.
• Kobo and ebooks.com — both sell some titles unlocked; look for the DRM-free label before buying.
• Internet Archive — scans and public-domain texts.

Your own library

Keep your books in Calibre on a computer, run Calibre-Web beside it, and Ledgable browses the whole collection over OPDS from Settings › Books. Or point every device at the same WebDAV folder, the library hub, and a book added on one device is offered on all of them. Adding books and comics has the steps.

Pages that become books

A long read on the web can be bound into an EPUB from The Net with Save as a book, pictures included. It lands on a Web shelf and reads like anything else you own.