Unless you're prepared to get technical, you can only read Amazon books on your Amazon Kindle paid for with your Amazon wallet.

To be fair to Amazon (which is not an opener I will use often), it isn't especially difficult to use their Send to Kindle web UI (www.amazon.co.uk/sendtokindle and so on), but it is almost as poorly publicised as their customer service options.

I buy a fair number of ebooks from independent publishers (like Verso or Subterranean Press, for example) and drag-and-drop an epub (or PDF, DOC, DOCX, TXT, RTF, HTM, HTML, PNG, GIF, JPG, JPEG, BMP !) is so much easier than emailing the file and then having to faff about because Kindle's email allowlist function doesn't seem to work.

But I definitely agree that the lack of cheap ereader devices is a problem.