rjnagle, in addition to the first three sites you mentioned, I also follow these blogs for free and cheap books:
1.
Books on The Knob
2.
RandomizeME
3.
Pixel of Ink (and its sister site
Inspired Reads)
In my experience, the first one is the most comprehensive with the second one following close behind. Pixel of Ink (and Inspired Reads) only features Kindle books.