If your interest is in the "2.The Book of Joe" specifically as an episode title (Season 13, Episode 2), it refers to a comedic plotline involving Joe Swanson. Detailed breakdowns are available on the Family Guy Wikipedia page.
: After helping his friend Joe Swanson write a children’s book, Peter Griffin takes over the project. When the book becomes a success, Peter takes all the credit, leading to a rift in their friendship. 2.The Book of Joe
: It balances the humor of Joe’s current predicament with flashbacks to the 1980s, exploring his first love and the tragedy that drove him away. 2. The Family Guy Episode: "The Book of Joe" If your interest is in the "2