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Amazon is making it impossible to remove the DRM from Kindle Books
(goodereader.com)
This is a most excellent place for technology news and articles.
Your "Send to Kindle" email address is in your profile details, under devices and apps. In the app (Android) it's under settings.
https://www.amazon.com/gp/sendtokindle
Sorry, I meant any more details about sending books to kindle via USB - I've never had any issue doing this.