I would definitely read that

Wasn't there a long Tumblr post at one point,where people were "writing" the character's lines or something?

The Neverending Story is sorta like that.

*has actually started a book like this tbh*

Sophie's world by Jostien Gaarder breaks down some interesting walls.

that book was already written...
Tom Riddle's Diary.

Between the lines by Jodi Picoult. Pretty good

Did you mean "The Neverending Story"?

Deadpool is the main character