Its just been a couple of years that I've gotten into audiobooks and Kindle. As someone who listens to audiobooks during commute and reads himself to sleep, I wish there was a feature to sync the progress on audiobooks and Kindle. Use case -- I read the book on Kindle till page 100 and start my commute for work. I open the app on my phone and the audiobook starts from page101. While returing I complete till page 150 and when I start the book again at home, it starts from page 151. Not sure if there are any apps that support this interoperability but it sounds like a very useful feature for readers who prefer both reading and listening to books.