Prodigal Son — Thomas B. Cavanagh

Three months after he's undergone surgery to remove a brain tumor, retired police detective Mike Garrity's cancer is in remission. It's great news, but when you've been planning to only live for a few months, you don't normally worry about savings. A return to health also means a return to the workforce, and whether he likes it or not, Mike's going to have to adjust to the role of private investigator.
He's also bounced back into a romance with a member of his cancer support group, Debbie Watson. But while Mike may be celebrating his victory over cancer, Debbie's just been handed a grim prognosis. Desperate, she asks him for help with what might be her last request: tracking down the son she put up for adoption twenty years ago.
Debbie's son is not the only lost child Mike has been asked to trace. A high school friend of his daughter has died of an overdose. The boy's father is convinced that the death was not a suicide and, though the case is officially closed, the officer in charge has his own doubts about its conclusion.
Back at work and stumbling headlong into a new relationship, Mike should have a lot to look forward to. But all is not as it seems, and he's about to find out that cancer isn't the only thing trying to kill him.