I don't think this was mentioned in this thread yet, but there is a good deal on the all-time classic science fiction book,
The Forever War, the multiple award winning novel from Joe Haldeman back in 1974.
Kindle version on sale for a quarter.
https://www.amazon.com/dp/B00PI184XG
Owning that qualifies you for the Audible version for $3.49 via whispersync.
http://www.audible.com/pd/Sci-Fi-Fan...ook/B002V0QDNU