Delete the contents of the 'My Kindle Content' folder within your 'Documents' folder.(Note: This option is not available in some Kindle for Mac versions.) Launch Kindle and under the menu choice 'Tools', 'Options', 'General', uncheck 'Automatically install updates when they are available without asking me.', and save.(Note: This is a 32-bit program and so it is not compatible with MacOS Catalina and later.) Older Kindle for PC/Mac version 1.17 can sometimes be installed to work around this problem. Method 1 - Revert to an older version of Kindle for PC/Mac The following descriptions apply both to Kindle for PC and Kindle for Mac unless one is mentioned explicitly. There are several methods for either avoiding or handling KFX format. Unless additional steps are taken, these books will fail to convert in calibre with the error: 'KFXError: This is an Amazon KFX book. Each book will appear in a separate subdirectory of the 'My Kindle Content' folder with a file extension of '.azw'. The Kindle for PC and Kindle for Mac programs use Amazon's KFX format for most books. There is also a new method (#6) using an old version of the Kindle for Android app described here. (Details in this thread.)This means that methods 1, 2, and 4 described below are no longer effective for those books. Important update: Amazon has made a change to their service so the books published since can no longer be downloaded using Kindle for PC or Mac versions prior to 1.39.