The Three-Cornered Halo is the 7th in the Inspector Cockrill series by Christianna Brand. It has dropped to $1.99 at Kindle and Kobo US (coupon/VIPable).
Kindle US:
https://www.amazon.com/Three-Cornere...dp/B005XE5AO4/
Kobo US:
https://www.kobo.com/us/en/ebook/the...-cornered-halo