If you could go back in time to give yourself one book, what book would it be and why?

(Inspired by this post)

I've had the experience a couple of times of happening upon something that would've absolutely been formative if I'd read it a little earlier. Wondering what books others wish they had read sooner!

I wish I could give my teenage self the Abhorsen books by Garth Nix. It would've become my personality 100%.