This Sydney Sheldon book surrounds the life of Toby, a normal guy with an abnormal…er…thingy and who can make just about anyone laugh and Jill his wife who married him so she could get her revenge on all the directors and show biz people who cruelly used her when she was trying to rise as an actress

Stranger in the Mirror (1976) is indeed a Sidney Sheldon novel about a comedian named Toby Temple and his wife Jill, who marries him for revenge against Hollywood figures who mistreated her. The plot description matches perfectly. The Day of the Jackal is by Frederick Forsyth, Windmills of Gods is also by Sheldon but has a different plot, and All the World's a Stage is not a Sheldon novel title.