The Kindle device is a bit weak in cockpit for reasons stated by others. However, I find the Kindle app on Android and iOS to be an excellent addition to my cockpit kit as doc reader since it keeps track of where you are in the doc, and it accepts PDF.
I convert all relevant docs to PDF (even from apps that dont provide pdf output, I use bullzip pdf print driver in windows -- eg for tripkits from jeppview, met briefings checklists, and any other handy references). I can store locally "on device" within kindle app, so works fine when away from network connection e.g. in cockpit when flyi
ng.