Until now, sideloading Kindle ebooks to your Windows Phone 7/8/8.1 device has been impossible. Now it is easy to not only sideload to the Kindle app, but have Whispersync functionality as well.
Whispersync allows you to sync the progress you’ve made reading a book you’ve purchased from Amazon between your Kindle and other devices that have Kindle apps. You’ll notice that when you copy your own .mobi ebook files to your Kindle and your phone, they won’t sync progress with each other, even if they are the exact same file. There is a very simple solution for this.