The Iago Factor — Tony Nash

Faced with one of his hated 'Thursday' murders - a young girl dressed in police uniform, fished out of the River Thames - John Hunter knows that with no obvious scene of crime he is on a hiding to nothing, but that one has to go onto the back burner while he investigates another murder - that of a senior police officer, Commander Freddie Sloane, whose real character, John finds, is far different to that of his upright public persona.
What John does not know is that there are many cross-connections: someone out there is determined to destroy his marriage to Jane, and false evidence of their infidelities is only the start. Jane is arrested for Freddie's murder, with enough evidence to make it appear a slam-dunk case. Her pistol, kept in their personal safe, fired the killing shots.
John is taken off the case, but continues investigating, receiving death threats from the Russian mafia on the way.
One of his chief suspects is the lover of a woman who runs a home for battered women, a woman who used to share his home and bed.
With new information, he realizes that she is in grave danger and hurries to try to save her, but arrives too late.
Her death does suddenly give him the killer's identity, but in the confrontation that follows he learns facts that make him want to 'look the other way' and allow the killer to go free. In this he is thwarted, and another death results, but Jane is seen to be innocent and is released.
Things get back to normal and John is satisfied with results, until he learns to his horror that far from being the prime mover in the investigation, as he so fondly believes, he has been played throughout like an outworn, one-string fiddle. Only his sense of humor saves him.