PR will certainly give a very good initial boost. It will increase popularity for sure. But an advertisement on FB and other platforms can boost the sales more. Now, both are technically related if you consider overall impact analysis. If you see an advertisement for a book, do you jump and buy? You may Google to know more and in that scenario, the PRs can help a lot. There can be companies falsely promising you exceptional results. But, to be honest, finally, the content is the King. If the book has very good content, it will be a winner in the long run.