The Four Johns — Ellery Queen

A teaching assistant knows that a man named John is out to kill him -- but which John is it?

Mervyn Gray sits at the college coffee shop scowling at his notes. The waitress assumes he's just another overworked Berkeley undergrad, but she is dead wrong. Gray is a teacher's aide fighting for his life. Two days ago, he was nearly poisoned. Yesterday, a bullet missed him by inches. Unless he can guess who is trying to take him out, and why, Gray will be dead by tomorrow. He has heard that his assailant is named John -- but there are too many Johns to choose from.
 
One John is a philandering librarian, another a lustful accountant. One is a disreputable fashion photographer, and the last is a half-baked campus poet. All four are in love with Mary Hazelwood -- and she has been missing for a week and a half. All Gray knows is that any of the Johns could be responsible, and he's the next target on the murderer's list.