The "Gray Man Series" is a collection of action-thriller novels written by author Mark Greaney. The series follows the character Court Gentry, a former CIA operative turned assassin, known as the "Gray Man" due to his ability to blend into any environment and evade detection. The books are known for their fast-paced plots, intricate espionage, and intense action sequences. The series began with the first novel, titled "The Gray Man," published in 2009. Since then, it has expanded to include multiple sequels, each featuring Gentry as he navigates dangerous missions and confronts various adversaries. The series has gained popularity for its realistic portrayal of covert operations and has been adapted into a film starring Ryan Gosling.