When the detective in the mystery has a manservant named Jock Strapp, you get a pretty good idea of what to expect. It's a detective story, well sort of as the hero is not all that honest himself, even when he's sober. So you combine the stolen paints, the secret police, a dead client, and the obligatory ravishing young widow -- and it's hard to make a dishonest living.
This is the first of a series of British mysteries beginning in 1972. It's written by an art dealer. But as the introduction says, This is not an autobiographical novel: It is about some other portly, dissolute, immoral and middle-aged art dealer. The rest of the characters are quite imaginary too, especially that Mrs. Spon, but most of the places are real.
Popular enough to have assumed almost cult status in England, I'm glad to see that it's finally crossed the pond.