You can "remove from device" (assuming it is a touch device hold down on the cover until a menu pops up) this means you will not see them if your app/device is switched to downloaded/on device (what it says depends on the device you are using) You can see them again and download if you wish if you switch to "all"
You can also go into "content and devices" in your Amazon account and delete them BUT if you do it that way they are gone forever and if you want them again you will have to buy them again.
or you coud create a Collection named Read and put them all in that Collection, then sort by Collections
and be careful, if you go the delete from account route. It's all too easy to accidentally delete a book you want to keep and there is no undo or undelete option. If you delete a book you didn't mean to delete, you will have to purchase it again unless you talk to Customer Service and find a very sympathetic representative who will replace it.