His Dark Materials trilogy by Philip Pullman. Fantasy. Okay - so no surprises here! But these books are just fabulous - definitely challenging for a struggling reader and at the upper end of the 7-13 age group - I would say a confident reader aged from 10 to 100!
“Every atom of me and every atom of you… We’ll live in birds and flowers and dragonflies and pine trees and in clouds and in those little specks of light you see floating in sunbeams… And when they use our atoms to make new lives, they wont’ just be able to take one, they’ll have to take two, one of you and one of me, we’ll be joined so tight…” ― Philip Pullman, His Dark Materials Trilogy: The Golden Compass / The Subtle Knife / The Amber Spyglass