"Golf is Not a Game of Perfect" by Bob Rotella. Short, easy read. It's an older book, you can probably find a copy on Amazon or something for a couple of bucks. He works with ball golfers, but none of his stuff involves technique. It's all about the mental game, and every bit of it translates to DG. This book helped me, as a guy with a low frustration tolerance threshold, to deal with the bad stretches in a round. It's mostly about accepting that you control your thought process and not the other way around, but you have to work at it just like your form. Most people don't have that mental discipline naturally, just like most people can't pick up a Tbird for the first time and launch it 300 ft.
In any dispute the intensity of feeling is inversely proportional to the value of the issues at stake.