Niels Bohr, the Danish physicist who invented rather a lot of Quantum Theory, said something along these lines: if you think you understand Quantum Physics then you don't. This book isn't easy, even if you are in the Sixth Form, but you wouldn't expect it to be, given what I've said about the topic. It is as clear an introduction to the topic as you'll find. Quantum Physics is wonderfully weird. It allows parallel universes and a lot of odd things. You won't understand it all whilst you're still at school but it will give you a great head start for future studies.
PS We've got more than one copy of this book - it's that important!