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What Amazon Kindle? Here's an Open Source eBook Reader
(itsfoss.com)
This is a most excellent place for technology news and articles.
Kinda basic for the price, and the built-in store is the B&N Store. But they at least have EPUB and PDF support, and Adobe DRM compatibility, so you can hook them up to a PC and transfer books.
They do run a stripped down Android underneath, so you might be able to Root them and install a third-party launcher and apps on it, I've done this in the past, but I haven't kept up in recent years(I've switched to Boox devices) so it might not be as easy to Root these days.