I once went as Tris from Divergent to a dress-up party. Apart from really liking the character/series, I went this route because of the simplicity behind the outfit. I wore black jeans, boots, and black tank top. I also topped it off with a grey cardigan to highlight Tris's ties to her original faction.
Another option could be to go as Thursday Next from The Eyre Affair by Jasper Fforde. She's a literary detective, so anything detective-y could work.
It's awesome that you'll be dressing up for World Book Day!
"I am only one, but I am one. I can't do everything, but I can do something. And I will not let what I cannot do interfere with what I can do."
- Edward Everett Hale