Unfortunately there is no money to be saved in what you plan on doing. When you enrol for a course you are paying for the manuals, progress tests, revision classes etc. It is unlikely that you can negotiate a discount from a school having already obtained another set of books. In fact the school is likely to say that it is obligated under its licence to give you their books as they have been audited and approved by the relevant aviation authority. Also if you end up with books from another school then it will mess you about being taught another syllabus or method.
In the whole scheme of things the ATPL theory is a minor cost.