Plan for Chaos (Modern Library edition) by John Wyndham is $1.99 at Amazon US and Kobo US.
https://www.amazon.com/Plan-Chaos-Jo.../dp/B09JBJHZ6N
https://www.kobo.com/us/en/ebook/plan-for-chaos-2
Meanwhile, on the other side of the ocean ...
The Chrysalids (Penguin edition) by John Wyndham is £0.99 at Amazon UK and Kobo UK.
https://www.kobo.com/gb/en/ebook/the-chrysalids
https://www.amazon.co.uk/Chrysalids-.../dp/B002RI9VGM