The book on Fleabay mentioned by paco has now gone but, for £15 that was an absolute bargain!
Even for the full price, that book will have everything you need to be getting started and it's a book you'd probably want to refer to throughout your flying career so not money wasted.
However, I'd recommend you get all your PPL exams out of the way before you think about extra-curricular studying for ATPL.
Cheers
Whirls
PS - no relation to Phil Croucher!!!