Visit Amazon official site and log in with your own kindle account, then go to "Manage Your Content and Devices" and select the "Settings" tab. To keep our kindle library in sync, we must turn the ''whispersync for books" service on. How to Turn on Whispersync Settings for Kindle Step 1: Enable Whispersync Device Synchronization on Amazon account. The official guide to sync non-amazon books across Kindle devics and apps How to turn on whispersync settings for Kindle