The three books by Stanislavski - "Building A Character", "Creating A Role" and especially "An Actor Prepares" are all known around the world as invaluable acting handbooks. But they're not for the faint-hearted!
They will need reading and re-reading. I still haven't finished them and i've had them a long time now!
But they offer fantastic techniques, advice and ideas that really are invaluable to actors everywhere. I would fully recommend these to anyone seriously involved in their A level or AS level Drama studies, because Stanislavski's pretty important to us!
But be prepared to have your brain taxed, and maybe have a slightly lighter, more "trashy" book to hand for when your brain just needs a rest :-p