If you have enabled iCloud Photo Library on your iPad, you should enable the Optimize Storage option under Settings > Photos. This will free up space on your iPad without deleting the photos.
Or you could connect the iPad with the lightning cable to your computer and navigate to it, move all files in the DCIM folder to your computer. This will also free up space.
As for the Kindle app, I don’t know, hopefully someone who knows will come along and help you out with that.