When you sign up for an ATPL course, the school will provide the required books. These will be included in the price of the course. So unless you can find a school which is prepared to give you a reduced price and let you use your own books, you will be increasing the overall cost of your training by purchasing books before signing up for the ATPL course.
Do you think it would be OK to use ATPL books to study for PPL theory?
If you already own a set of ATPL books, and if you are actually interested in the subjects and like learning ( many people are not and do not), then the best approach would be to use your PPL books as your primary study material. Then consult your ATPL books for any subjects where you are unsatisfied with the PPL material.