When I studied, I had the current RxPrep, but I also had the previous years as a PDF. The PDF was nice to skim random chapters during my BS rotations without needing to carry the book around. The biggest issue with going with an older book is if there were any major guideline changes. However, I am not sure exactly how far behind major changes that the NAPLEX usually follows. Also from my experience, the NAPLEX does not chase perpetually changing guidelines like HIV, which seem to change every 6 months. Not to say they do not quiz over HIV, because they definitely do, it is just more generic questions about major SE and CI for their use.