Just sitting there in the goodwill….
WHOA WHOA WAIT WHOA.
No, wait, hold up, I got it.
Jack developed feelings for Rose the first time they met. However, he knew the Doctor liked Rose too, so he’d be in BIG trouble next time they met. So, in order to express his feelings, he went back in time to where Nine couldn’t follow him (because timey-wimey) and wrote this book.