Why Didn't They Ask Evans? is a non-series titles that is one of my favorite Agatha Christie books. (Actually, several of her non-series books are among my favorites of all her books...)
I especially like the interplay between Bobby and Frankie as they figure out what's going on, while dealing with various mildly disapproving parental figures. It kind of reminds me of some of Heinlein's "juveniles", which are still great fun for adults to read too. In any case,
Evans has dropped to £0.99 at Kindle UK as part of today's Daily Deal.
Kindle UK:
https://www.amazon.co.uk/dp/B0046A9MZO
Kindle UK/Smile:
https://smile.amazon.co.uk/dp/B0046A9MZO