Most times these books are only free for a day or two, so you have to grab them while you can. Worry about whether you really want to read it later if it seems interesting. And be careful, they can change from free to not free at any time. I have also decided to list books for no more than 2 days in a row to avoid cluttering up the posts, unless there is some reason I think the book deserves extra attention.
From the top 100 free list
Sargasso of Space (Solar Queen Series)
[#11 - 2nd day - from one of the legends of science fiction of the 1950s.]