A true diversity of cheap Kindle books right now available:
Wonders of the Universe (Wonders Series) by Brian Cox and Andrew Cohen. I’m not sure if this is the ideal Kindle book because the original print version is full of illustrations that may or may not translate well, but that might depend on what you want to do with it. I like having print copies, if I get them cheap, that go along with excellent documentaries (and this is a book that goes along with an excellent documentary), and a kindle version might be fantastic for that since it is searchable, and the whole idea is to have a memory jogger or a reference.
Bury My Heart at Wounded Knee: An Indian History of the American West by Dee Brown is a classic. It was one of the key early books (ca 1970) to revise, as in making more accurate, the American conception of the history of Native Americans. The Kindle version contains additional information not found in the original.
If you read the Chroncles of Brother Cadfael and are not up to book 14, you may want to grab The Hermit of Eyton Forest , by Ellis Peters, which is book 14 in that series!