What is the episode that Mcgee writes a book on NCIS?

The episode you're thinking of is "One Last Score" from Season 10 of NCIS. In this episode, Timothy McGee actually writes a novel based on his experiences as a field agent with the NCIS team, though he changes the characters and situations to protect the team and himself.

He ends up finding success with the book, causing a bit of a stir within the team and even prompting some real-life NCIS agents to come to him for advice. It's a fun episode that plays with the idea of blurring the lines between fiction and reality.