Well, I think Jackson can help to understand the concepts behind Purcell's book better, but I'm prejudiced, because I don't like Purcell's book, because I think he overcomplicates the relativistic foundation of E&M rather than simplifying it. I think rather than reading Purcell you should take Schwartz and Landau&Lifshitz vol. II for the modern relativsitic approach. Jackson is a comprehensive more conventional book in the tradition of 20th century textbooks, emphasizing the non-relativistic view and only in the later chapters introducing relativity. Nevertheless, Jackson is way better than Purcell concerning the foundations.