Or rather, they decided once he died that it was time to wrap it up.

Leonard seemed committed to writing about the Raylan character once the show hit the air. His final novel, "Raylan," while awful, was essentially an idea factory for "Justified." Yost and crew pulled seasons worth of characters, episode ideas, plot developments, and other minutiae out of it (and made it better each and every time). So at the time of his death, Leonard was working on ***anotheer*** Raylan Givens book.

And once Leonard did die, they used his other books for inspiration; the John Tucker character in the final season was lifted out of one his early books.