Amazon / Penguin has Frank Herbert's classic
Dune on sale for $1.99 today. As usual this may apply to other vendors as well since this is sold by the publisher so you might want to check if you prefer someone other than Amazon.
Amazon also has
The Forever War by Joe Haldeman on sale for $1.99, which is also a good read.
Dune:
https://smile.amazon.com/Dune-Frank-...dp/B00B7NPRY8/
The Forever War:
https://smile.amazon.com/Forever-War...dp/B00PI184XG/