This book is unlike any on its subject I have ever encountered. Even though the author of this book passed away in 1953, it has a clearness and depth directed towards understanding the essentials of flight that make it a worthwhile - and very inexpensive - read today. Of course, to supplement this book, given the progress made in the field since the author's death, you'll require several textbooks.
Part one is entitled "Equilibrium and Steady Flow in the Atmosphere". Chapter one of that section, though correct in its physics, is simply not applicable to high-speed or turbine-powered aircraft. The other chapters in part one, though, are a great introduction to the dynamics of incompressible fluids that are still completely relevant.
Part two is entitled "The Airplane Wing", and contains material that the author not only expertly compiled, he actually contributed much of this material to wing theory himself. Again, you'll need supplementary material to be current on this information.
Part three is entitled "Propeller and Engine". Where propellers are still in use, the information in this book is correct. The basics on the airplane engine are here, but again time has marched on as to the forefront of technology.
Part four on "Airplane Performance" talks about performance problems in propeller planes. You'll need other material to understand how the jet engine affects performance.
Part five on "Airplane Control and Stability" contains great sections on the theory of non-uniform flight and also a very clear introduction to the theory of dynamic stability. Information on automatic controls is missing, though.
I'd say read this book and use it as a reference as a basis for understanding the mathematics and theory of flight. Use it as a firm foundation for learning about the more modern aspects of the field. One thing I really appreciated about this book is how it is essentially self-contained. The author develops the mathematics as needed, and it is generally well-illustrated, which is generally not a feature of older engineering textbooks. There are problems at the end of each chapter. Unfortunately, though, answers are not included. It is definitely geared for the upper-level undergraduate engineering student that has already finished physics, calculus, statics, and dynamics. Highly recommended.