I just got an e-mail from Thrifty Homeschoolers about this. It's a bit late in the evening so I hope some of you get this in time.
If you go to the Kindle store search for Kaplan (see below) you will find lots of freebies! I'm going to get back online after putting the dc to bed and get the SAT subject test for Chemistry for sure!
1 - click on the link: http://tinyurl.com/4h8hfy2
2 - In the search box change it to Kindle Store to narrow it down (or just browse there for the freebies)
3 - Find what you want. Get it for your Kindle.
4 - If you don't have a Kindle (which I don't, btw, but sure wish I did): In the green box on the right-hand side that says "buy now for 1-click" (DON'T click that) there is a place to click that says "available on your PC." That's what you want to click if you do not have a kindle.
ALSO: I did another search in the kindle store for just "$0.00" in the search box and found more for free, some public domain and some not.