So far I have only browsed the history offerings (not gonna lie - I started with what I loved). I went ahead and made a wishlist (and made it public) just to keep track of some of what I was finding. I am not going to vouch for the worthiness of any of these books. Some I briefly downloaded to check out, but most I didn't. If you're interested, you can see that list here. I will keep adding to it regularly (adding books to a wishlist is like compulsive shopping, yes? It's just cheaper!).
Just as a sample, a publisher called Charles River Editors has hundreds of books, most of which are available on Kindle Unlimited. Each is about 45-70 pages, and they cover many topics ranging from the history of various Native American tribes, to ancient cities and civilizations, to the history of Iraq, Syria, and ISIS. An example of one of the books is below.
What about The Only Grammar Book You'll Ever Need? Sounds good to me! How about SAT Prep? The SAT Prep Black Book: The Most Effective SAT Strategies Ever Published is also there.
Honestly, there are so many books available that the sky is the limit. Also, they seem to be adding new books every day. I think that Kindle Unlimited is the best deal around. No, there are no affiliate links in this post - I am just a bibilomaniac - er bibliophile - and I want to share that obsession (I mean love!) with everyone I can!