I certainly found it useful to return to PPL work (actually I sat the JAA exams, as this was February 2000 and the whole thing was new). Although I did so by sitting the 4-week course offered by the sadly-demised SFT Aviation, if you are good at lone study (I am not!) then Trevor Thom is an excellent series of books to work through with a content beyond the PPL syllabus. I have not used them but the Pratt books have a good reputation too.
The whole PPL syllabus is included in the ATPL in most subjects. The exception might be Air Law, with the PPL being UK law, and the ATPL primarily covering ICAO regulations. However studying this will never be wasted, as you need to know you are flying legally!