I didn't buy many e-books from Amazon for my briefly owned Kindle. However, Amazon has by far the widest range and you can still buy Kindle books via the Kindle app to read via PCs, Macs etc.
Read first Apprentice Alf's blog, especially Ebook formats, DRM and you
Then the plugin tools (including instructions) for Calibre are linked from here: https://apprenticealf.wordpress.com/2010/02/11/hello-world/
It works!