I am trying to find the title of a book where a guy thinks that he is a private detective in the 40s or so.?

He apparently has amnesia and he thinks that he is a detective or a gumshoe. He constantly talks as if he is in those times calling women dames and wears the suits with the big lapels and the two tone shoes. He gets a couple of cases. One where he is trying to find out if this woman's husband is running around on her. He hires a secretary who realizes that he is a little strange, but she needs the money so she stays with him. One of his clients ends of giving him a lot of money to stay in business and he ends still thinking that he is this gumshoe private detective. It is a mystery, but humorous.