Donald Hamilton

Matt Helm is an excellent secret agent character. The books are formula but always entertaining.

Warren Murphy and Richard Sapir

The Destroyer series is fun. Once they started writing them using a "book mill" they deteriorated, but the early books in the series are excellent. There must be more than 100 books by now.

Don Pendelton

Mack Bolan. The Executioner, turned into several spin-off series. The early books were excellent, deteriorating when the went into the "book mill" format.

Mickey Spillane

Mike Hammer, private eye, is a great, tough character. All of the books are good.