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You don't have to delete the books. Books don't take much storage. Movies are the ones that are taking up most of all the storage. Follow the steps below to remove the movies from your Kindle Fire.
If you want to delete videos from Gallery, then from home screen select “Apps” > “Local” > “Gallery“. Tap and hold a video you wish to delete. The Delete option will appear at the bottom of the screen. Check any other videos you want to remove, then tap “Delete“.
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