I think that even book sales may struggle to find good homes for books these days: our own local library seems more likely to be selling off CDs and other digital format items...!
In was it helps anyone else, here is a notice I had from Amazon a few weeks ago when trying to import a DRM protected item to my kindle:
Send to Kindle can convert and deliver the following types of documents:
EPUB (.epub)
Adobe PDF (.pdf)
Microsoft Word (.doc, .docx)
Rich Text Format (.rtf)
HTML (.htm, .html)
Text (.txt) documents
Compressed documents (.zip, .xzip)
JPEG (.jpg), GIF (.gif), Bitmap (.bmp) and PNG (.png) images.
If the format of the failed document is supported (listed above), please ensure that the document is not password-protected or encrypted, then try sending it again.
Other helpful tips for sending documents to Kindle via email:
Limit attachments to fewer than 25 documents per email.
Individual file size should be less than 50 MB (before compression, in a ZIP file).
To learn more, please visit our Help page. By downloading or using Send to Kindle, you agree to the terms
https://www.amazon.com/gp/help/cust...d2Ouy8c6i&ref_=pe_1825130_138612630. Amazon Kindle Support