Believe it or not, before I was a tennis player, I was a piano player. I would be the first to admit that I don't like playing the piano nearly as much as I like playing tennis. But I started to learn my first tennis lessons in the piano studio.
You might be asking yourself, "How can anything about the piano teach you about tennis?" Actually, tennis and piano are very similar mentally. Learning how to prepare for recitals also taught me a lot about preparing for matches. I would use the same techniques I used to sit down in a piano recital, knowing I could play the piece perfectly, as I would on the tennis court, knowing I could step out on a court and win.
To gear us up mentally for piano recitals, my piano teacher would often quote from W. Timothy Gallwey's book The Inner Game of Tennis: The Classic Guide to the Mental Side of Peak Performance. This book is a must read for any tennis player, and to be honest, is a great read for anyone interested in improving mental performance. Here is a link to Amazon or Barnes and Noble where you can purchase the book if interested.
So what makes this book so good? The premise of the book is that the mind is often the biggest obstacle we have to overcome when playing tennis (or doing a lot of things in life). Gallwey's first goal is to convince the reader that the mind often limits us from reaching our full potential. 
After we understand that fact, as players we know what aspects of our game we need to work on. Gallwey then breaks down the many ways that we can work on using the mind as a tool for success rather than a having it be limiting. For example, he identifies that "the first skill to learn is the art of letting go the human inclination to judge ourselves and our performance as either good or bad. . . .When we unlearn how to be judgmental, it is possible to achieve spontaneous, focused play." He explains that not being judgmental doesn't mean "ignoring errors." He just teaches that observing our performance without getting caught up in positive or negative judgments can help us improve skills.
As I have coached tennis, I have noticed that the mental side of tennis is often the most debilitating and the hardest to overcome. This book is phenominal at helping players understand what is really going on in the mind, and the tactics for overcoming these mind games work.
Whether you feel like you are in control mentally on the court, or you feel like it is your biggest challenge to overcome, your time will not be wasted if you decide to pick up this book. In addition, you will find yourself pondering the principles in other aspects of your life. You stick your foot in your mouth, and all you can do is replay the incident in your mind, telling yourself how stupid you were. Gallwey would tell you that when you just keep judging the situation, you may paralyze yourself for the next conversation you have. He would advise just observing in your mind what happened and planning a different approach for the next conversation.
Gallwey says, "Take off our judgmental glasses, whether they're dark or rose-tinted." That's good advice on the court, in the office, at school, or at home.