I wouldn't counsel getting the PMBOK 6th edition unless money's no object. The Process Groups Practice Guide (PGPG) is essentially the core content from PMBOK 6th edition that isn't in PMBOK 7th edition, in a more up-to-date text. However, the PGPG might not be of the greatest use in preparing for the PMP exam, either. It's good for getting to know the PMI's official processes, which might be good in building one's knowledge and skills as a Project Manager... but that's not the same thing as passing the PMP exam!
Do you already have the Agile Practice Guide? If not, I'd advise getting that one first. In the course that I followed, the training school's two core texts were PMBOK 7th edition and the Agile Practice Guide. The PGPG hadn't been published when our course started, but I bought an electronic version a few months before my exam. On reflection, I don't feel that having read the PGPG contributed to my success in the exam, whereas the Agile Practice Guide was pivotal.