Dealing with debtors turns deadly for a prickly PI in this hard-boiled mystery by the creator of Perry Mason and author of Bats Fly at Dusk.   A hot-headed widow and a glass-jawed ex-lawyer, Bertha Cool and Donald Lam seem like an unlikely duo of private detectives. Even so, they've managed to solve the most difficult of mysteries--when they're together. With Donald now on a European vacation, Bertha is hesitant to accept any new business--but money is money, and this new case seems routine enough . . .   Bertha is hired to get sales engineer Everett Belder out of a $20,000 problem. Unfortunately, his troubles soon multiply. His wife is receiving poisoned-pen letters accusing him of infidelity. Then she disappears. And there's also the matter of the body in his cellar. With everything spiraling out of control, Bertha must determine who is behind this deadly game of cat and mouse before another murder comes into play.   "No one has ever matched Gardner for swift, sure exposition." --Kirkus Reviews   "The best American writer, of course, is Erle Stanley Gardner." --Evelyn Waugh