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!
Philip Pullman's The Golden Compass, His Dark Materials book I, was the 1996 winner of the prestigious Carnegie Medal, England's highest honor for children's literature. This text is from his acceptance speech for that prize.