how do I close out from a book in Kindle?

Tap on the little House icon on the top left corner of the Kindle screen to go back to the book list.

If you mean go back to the beginning, tap the book icon in the uppers left an select Cover or slide the round button at the bottom all the way to the left.

To totally leave the book and go back to the list of your books, tap the House icon in the upper left.