The Door Between & The Devil to Pay by Ellery Queen (Good, 1980, Pbk, 192 pages, Signet)

Used in good condition: cover and pages have usage wear; pages are lightly discolored. 

Ellery Queen, the gentleman detective, is a handsome Harvard grad who works as a private eye alongside his father, Inspector Richard Queen of the NYPD.

In The Door Between, they face one of their most daunting cases. Karen Leith is dead. An acclaimed novelist turned recluse, she died alone, in a small, secluded room of her odd Greenwich Village house. Reportedly tormented by a scandal from decades past, she killed herself. Or did she? The deeper Ellery Queen delves into her past, the more certain he becomes that she was murdered -- in as clever and horrifying a manner as he has ever encountered. Queen's superb powers of deduction are put to the ultimate test in this intricate whodunit.

In The Devil to Pay, stocks rose, stocks fell, but Solly Spaeth always came out ahead. The fat little man with the computer mind was a financial magician - a wizard at making other people's money disappear into his own pocket. Then Spaeth made one fast deal too many, and someone decided to make a different kind of killing. By the time Ellery Queen arrived on the scene, Solly had cashed in his blue chips, everyone including his loving mistress and his hating son was a suspect - and Ellery had no margin for error as he plunged into a game of wits with a murderer too deadly to sell short.