The red horse : a Billy Boyle World War II mystery / James R. Benn.

Enhanced descriptions from Syndetics:

England, 1944- Recovering from physical and psychological wounds sustained in the liberation of Paris, US Army detective Billy Boyle and Lieutenant Kazimierz aresent to a convalescenthospitalin the English countryside-only to discover that St. Albans may have its own war secrets, including a killer.

"As historical detective series go, this one is extremely well tended by an author who clearly dotes on his hero. As do we."- The New York Times

Just days after the Liberation of Paris, US Army Detective Billy Boyle and Lieutenant Kazimierz are brought to Saint Albans Convalescent Hospital in the English countryside. Kaz has been diagnosed with a heart condition, and Billy is dealing with emotional exhaustion and his recent methamphetamine abuse. Meanwhile, Billy's love, Diana Seaton, has been taken to Ravensbrück, the Nazi concentration camp for women, and Kaz's sister, Angelika, who he recently learned was alive and working with the Polish Underground, has also been captured and transported to the same camp.

This news is brought by British Major Cosgrove, who asks Billy for help, unofficially, in solving what he thinks was the murder of a British agent recuperating at Saint Albans. The convalescent hospital is really a secret installation for those in the world of clandestine warfare to recover from wounds, physical and emotional. Some are allowed to leave; others are deemed security risks and are detained there. When a second body is found, it is evident that a killer is at work in this high-security enclave. Now Billy must carry out his covert investigation while maintaining his tenuous recovery, shielding his actions from suspicious hospital authorities, and dodging the unknown murderer.