[sufue]

The Crooked Wreath (apa Suddenly at His Residence) is the third in the Inspector Cockrill series by Christianna Brand. It has dropped to $1.99 at Kindle and Kobo US and possibly others - and it's couponable/VIP-able at Kobo.

Kindle US: https://www.amazon.com/dp/B005XE5BHK
Kobo US: https://www.kobo.com/us/en/ebook/the-crooked-wreath

Spoiler:

A family patriarch is murdered on the eve of signing a new will
Sir Richard’s family has spent years waiting for him to die, but despite his weak heart, the old man simply refuses to cooperate. In the meantime, he makes their lives miserable by changing his will every few months, depending on which of his strange brood he favors that moment. Now he calls them together to announce his most diabolical revision yet: complete disinheritance of all the wastrels who bear his name. But he never gets a chance to sign the papers—by morning, he’s dead.

Scotland Yard sends Inspector Cockrill, the only detective clever enough to unravel the family’s tangle of jealousy and deceit. Each member had reason to kill Sir Richard, but which one plunged the syringe of poison into his heart? With a family this mad, nothing is as complicated as the truth.