"The Principles Behind the Rules of Golf" (by Richard S. Tufts, and updated for the 2012-2015 Rules by William J. Williams, Jr.) is a fine book that costs a whopping $2 from the USGA.
This thread exists for two purposes.
The first purpose is for the open discussion of the book, the principles explained therein, and so on.
The second is to serve as a request that certain members demonstrate proof that he has purchased and read this book before creating any new Rules of Golf discussion threads on this site. The URL to this thread will be PMed to members as a means of requesting this of them.
Edit: Exceptions may be made, but unless you've been granted one, you must own the book to participate in this thread.