Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
You can delete unwanted apps or the books you read already. To delete, tap and hold down your finger on the item you would like to delete (in home screen) and tap Remove from Device. This will delete it from your device, but they can be redownloaded again from Cloud.
Refer this article -> http://www.lovemyfire.com/delete-from-kindle-fire.html
If you have any questions, please let m know.