Charlie Bone And The Invisible Boy

Description for CHARLIE BONE AND THE INVISIBLE BOY by Jenny Nimmo:

Ollie Sparks needs the help of Charlie Bone!

A new semester is beginning at Bloor's Academy, and danger is lurking around every corner. First Charlie's beloved Uncle Paton disappears on a long and perilous journey. Then Charlie and his friends make an astonishing discovery: an invisible boy, Ollie Sparks, has been imprisoned in the attic of the academy.

Charlie knows it's up to him to use his extraordinary gifts to help his uncle and Ollie. But lately, his powers have been changing--and not for the better. Nothing is what it seems, and Charlie doesn't know if anyone can be trusted--including himself...

This is #3 in Children Of The Red Queen Series.