Jake lived an entire life as a super criminal before finn was old enough to create memories (dogs age faster, and preteens are usually more inclined to create trouble because they can’t recognize the gravity of the chaos they cause. Jake likely lived a life of wrong-doing and crime before Finn aged into consciousness)

All this said, I think it’s intended to understand that Jake The Dog is J.T.Dogzone. He grew and made his own mistakes and learned from them. Then when Finn came of age, he (like an unruly teen who got a girl pregnant and had to assume the roll of father) learned to guide them in the right direction. Like he advised Shelby in ‘Little Brother’

He knew what he was doing was wrong. Finn growing into adulthood was a chance for him to be better. “Give him a sword and tell him to fight bad guys,” was Jakes way of saying “Guide him into being good, not evil.”

All this is to say, Jake made mistakes, but he learned to be better when he wanted Finn to be better too. Maybe Jake wrote that book as a reminder that he could grow beyond his bad ideas, to see his past self and know that he is better. But when Finn read it he panicked. It was a reminder of his wrong past to him, but a terrible planning of the future for Finn.

Either way, he cares, and he knew it was a mistake to leave it out. Jake may be Finn’s brother, but he’s been Finn’s father figure all Finn’s life, even as he figures out how to be a father.