Cover by Jim Manos, about whom Google turns up nothing, but a James Manos, Jr., is a writer-producer for movies & TV, including The Sopranos, The Shield, and Dexter.

Pinnacle, paperback; 1st printing, Sept 1983. UK hardcover: Collins (Crime Club), 1980; US hardcover edition: Doubleday & Co., 1980.

   From the back cover:

Famed for his extravagant houseparties and his nasty jokes, the Maharajah of Bhopore had asembled as odd assortment of guests for his annual April Fool’s Day fun. With consummate charm, he plies them with luxuries — and then, he tortured them with his tricks.

But mirth turned to murder with the crack of the Maharajah’s backfiring rifle. Obviously someone in the palace had a deadly sense of humor … and Detective Superintendent of Police Howard was just the man to find out who the joker was.