Singularity — Susan Hart

Singularity is about Peter von Rabenhorst's recovery after he killed a man. He'd never killed a living being in his life and it gave him a heart attack. The thing about it, as he told FBI agent Lili Foxworthy, was that he liked it. Lili missed Peter's lovemaking terribly and it was taking him a very long time to recover. She brought in a psychiatrist to help her husband, but it only made things worse; far worse, because Dr. Hadrian Green has known the von Rabenhorst boys for a very, very long time.