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)
Almost half a century ago, renowned science fiction and fantasy author Andre Norton introduced apprentice cargo master Dane Thorson in Sargasso of Space and Plague Ship.
Dane signed on with the independent cargo ship Solar Queen looking for a career in off-world trade.
In Sargasso of Space, the Solar Queen free traders win exclusive rights to trade with the planet Limbo, but the crew arrives to find most of the planet's surface charred, with little signs of life. They find a valley with life, but others may still lurk. Worse yet, a strange force threatens to cripple the Queen. They must solve the planet's mysteries if they hope to escape not only with tradeable goods, but their lives.
[#4 - from one of the legends of science fiction of the 1950s.]
Shadow Bound (Shadow series)
[#14 - 2nd day.]
JET (International Action / Adventure Thriller)
[#90 - 2nd day. From an author regularly on the top 100 free list. This one has been free several times before that I recall. I have yet to read it.]