The Little Sister is the fifth in Raymond Chandler's Philip Marlowe series. It has dropped to $2.99 at Kindle US and Kobo US. It's normally priced at $9-ish and up, so if you want it, grab it!
This was part of an Chandler omnibus that was on sale in the UK a while back, so if you travel, check carefully before you buy...
Kindle US:
https://www.amazon.com/dp/B000FBFM4S
Kobo US:
https://www.kobo.com/us/en/ebook/the-little-sister-3