Music-making is a skill which fully involves both the mind and the body. It requires not only careful awareness and understanding, but an appropriate physical response and disciplined use of one's whole body. This implies that preparing to make music entails both physical and mental preparation. Madeline Bruser does a fine job of showing how to do both. While her book is called The Art of Practicing, nearly everything it says applies to performance as well as practice. The primary focus of the book is on playing the piano, but there are also many details on playing other instruments and singing.
The advice which Bruser gives covers awareness of the music, physical preparation and coordination, and the performer's emotional state. While the book isn't a philosophical one as such, it builds from and illustrates the philosophical maxim that one's thoughts, emotions, and physical state should make up an integrated whole.
On the strictly physical side, it covers stretching exercises, posture, breathing, and some mechanics of holding or positioning one's hands on the instrument. As she notes, tension interferes with performance, creating a harsh and mechanical performance if not outright mistakes. (One problem which I do have with her physical discussion is that the stretching exercises she offers are largely beyond the ability of normal human beings; but it's easy enough to pick the easier exercises or devise others. The object is to have a relaxed, extended body, not to tear a ligament.)
The mental side -- awareness and understanding of the music -- includes the ability to perceive the music directly and freshly, not just as abstract notes in the mind. Thus, it overlaps into the physical; Bruser stresses awareness of the vibrations in one's body and resonance in the air, as well as kinesthetic response to the music. Phrasing and articulation also play an important role in Bruser's view of musical understanding. Her discussion of this subject revitalized my own musical listening; apart from any effect on my ability to play the keyboard or sing, I immediately found I was hearing music more vividly and intensely.
One's emotional response to music grows out of both mental and physical factors, and is hard to isolate from the other aspects. Bruser discusses fear of failure, repression, confidence, and joy. She stresses the need to feel one's emotions, whether they are pleasant or not, and gives useful advice on how to deal with them.
There are many anecdotes about students and professional musicians, illustrating their responses to difficult situations. These illustrate many of her points, and sometimes give warnings; her citation of Leon Fleischer's obsessive practicing in spite of pain, which ruined his right hand, particularly struck me since Fleischer's recordings introduced me to the Beethoven concertos when I was small.
The chapters include question-and-answer sections based on seminars which Bruser has taught. These give her an opportunity to further clarify her discussion, and to show how the methods apply to unusual situations.
This is a book which you should read with your instrument close at hand, or (if you're a singer) in a situation where you can conveniently try out your voice. Be ready to pause frequently in your reading and try things out. The Art of Practicing can be an extremely rewarding book, if you are willing to put in the effort it demands.