Next paragraph in the book:

What is wrong with "Theory of Flight," from the pilot's point of view, is not that it is theory. What's wrong is that it is the theory of the wrong thing – it usually becomes the theory of building the airplane rather than of flying it. It goes deeply – much too deeply for a pilot's needs – into problems of aerodynamics; it even gives the pilot a formula by which to calculate his lift! But it neglects those phases of flight that interest the pilot most. It often fails to show the pilot the most important fact of the art of piloting – the Angle of Attack, and how it changes in flight. And it usually fails to give him a clear understanding of the various flight conditions in which an airplane can proceed, from fast flight to mush and stall. This whole book, and especially its first chapters, are an attempt to refocus "Theory of Flight," away from things that the pilot does not need to know about, and upon the things that actually puzzle him when he flies.