Nightmare in Pink by John D MacDonald

Travis McGee looks up the sister of an old war buddy.  Her fiancé has been killed and Travis is set to find out who did it and why.  The sister is convinced he was embezzling money from the investment bank where he worked.  She even has $10,000 that was hidden in his apartment.  But it looks too slick to McGee and yet missing obvious evidence.  The man who was supposed to be embezzled from recently had a breakdown and spent a month in a hospital, then when he was released, he moved out of his home and started living in an apartment with his secretary and his lawyer.  When McGee meets with the secretary for a drink, she slips him a rufee and when he comes-to he is in a hospital, hallucinating.  McGee figures out that the same thing must have happened to the banker.  He causes a ruckus at the hospital and escapes.  The lawyer, the mastermind behind the real embezzling, marries the secretary to prevent her from testifying against him and the banker is not able to testify to anything because he was lobotomized in the hospital.