Just as a kind reminder, PR is not a subset of marketing.
PR, Public Relations, is a term used to describe the activity responsible for managing organisational communication. This managerial venture should also generate a mutually beneficial environment for all parts involved in the communication process and in all spheres it will operate - internal, external, symbolic... Heads of PR, in theory, should report directly to the CEO of a company, with no intermediaries. In the largest companies in the world, this is how it is done. In the corporate world, PR is the very same as Corporate Communications. So it should not be treated as a tool of Marketing.
When PR is considered purely as media relations and does not explore its full potential, much is left aside on what could be achieved with the powerful and broad umbrela of Public Relations.
I still get surprised to see that among the Marcom community, not many people truly understand the essential difference between Marketing - the study of market (product/service, price, promotion, distribution, placing, merchandising, consumer's behaviour, USPs, etc) and PR - Social Communication (based on the Sciences of Communication, with a full understanding of the human communication process - encoding/channel/message/decoding - and behaviours, based on Communication theories, neuro-science, psychology, anthropology, sociology, cultural studies, philosophy).
Communication is a science and PR is a fruit of that science.
To avoid mistakes, it is always useful to read. Very important to remark, if you are within Marketing and want to understand Communications, it is needed to visit the Communications Sciences' literature to have a feel of why Communications' courses usually are much longer than Marketing. I stayed 5 years in University to complete my first Bachelor's Degree in Social Communication - specialisation in Public Relations. Glad I made this choice.
I understand the principles of marketing. But that did not happen by chance. I did some minors when in the University and worked in many marketing departments but, more importantly, because I read a lot about the area - books, scientific articles, posts, etc.
To name a few authors that I read extensively during my University years (which were 12 years in total in 5 countries), I would surely mention Philip Kotler and Peter Drucker - ahhh Drucker, what a genious!!! But I have to be honest, I like Public Relations 1000 times more than Marketing... PR is in my DNA.