”The 15.18.5 update is also having an adverse reaction to sideloaded books. If you deliver a book using Send by Email or copy it to your computer via USB, a critical issue may arise, where a pop-up appears with an ‘Invalid ASIN‘ number. The new DRM system is attempting to locate the book in the Amazon store to decrypt it, but since it can’t find it, it reports that the book is invalid. Amazon claims they are working on the issue, ... ”https://news.ycombinator.com/item?id=45393505