The Book on Your Game
If you can't write a book on your game, either you don't know your game or you don't have a game. It's as simple as that.
Most likely, if you are reading this tip and are at least a semi-serious player, you do have a game. So what is your game? What are your strengths, weaknesses, and in-between ones? How do you serve, return serves, and rally to get your strengths into play and dominate, while avoiding using your weaknesses? When do you use your other shots (shots that are neither strengths nor weaknesses, but something in between)? While you don't want to tie yourself down on what you'd do in any given situation, you should have a specific repertoire of shots you favor in any given situation, and be able to pick and choose from among those shots in any given situation, based on your opponent's game. Above all else, how do you dominate a match?
Not everyone's a writer, and few of you are actually going to write a book on your game. But you should be able to at least outline the book on your game, and have the entire book in your head. If you can't, then perhaps you need to think about your game and get to know it. Or, if you are new to the sport, develop your game, and get to know it as it is developed.