The 1943 printing of "The Analysis and Synthesis of Linear Servomechanisms" was classified as restricted and distributed accordingly. After the close of WWII, the book was declassified in order to permit general dissemination of the material.
From the Preface:
This book is a formulation of a servomechanism design procedure based primarily upon an analysis of the system response to sinusoidal inputs of various frequencies. Although a knowledge of the transient performance of a servomechanism provides an excellent basis for predicting the response of the system to the conditions of a particular application, the complexity of most physical systems makes the computation of the transient performance and the translation of the resulting information into physical design criteria extremely laborious. On the other hand, knowledge of the response of the servomechanisms to sinusoidal inputs is not quite as useful in enabling a prediction to be made of the servo response to the conditions of most practical applications, but the computation of the sinusoidal response and the translation of this information into useful design criteria are much simpler.