The complete open textbook is Gelb, A., and W. E. Vander Velde. Multiple-Input Describing Functions and Nonlinear System Design. McGraw Hill, 1968. (Courtesy of Authur Gelb and Wallace Vander Velde. Used with permission).

Individual chapters can also be downloaded below: 

• Chapter 1. Table of Contents, Nonlinear Systems and Describing Functions (PDF - 2.3 MB)
• Chapter 2. Sinusoidal Input Describing Function (PDF - 2.6 MB)
• Chapter 3. Steady-state Oscillations in Nonlinear Systems (PDF - 4.3 MB)
• Chapter 4. Transient Oscellations in Nonlinear Systems (PDF - 1.7 MB)
• Chapter 5. Two-sinusoid-input Describing Function (PDF - 1.9 MB)
• Chapter 6. Dual-input Describing Function (PDF - 2.8 MB)
• Chapter 7. Random and Other Signals in Nonlinear Systems (PDF - 3.1 MB)
• Chapter 8. Nonoscillatory Transients in Nonlinear Systems (PDF)
• Chapter 9. Oscillations in Nonlinear Sampled-data Systems (PDF - 2.4 MB)
• Appendix A. Amplitude-ratio-Decibel Conversion Tables (PDF)
• Appendix B. Table of Sinusoidal-Input Describing Functions (PDF)
• Appendix C. Table of Dual-input Describing Functions (PDF)
• Appendix D. Table of Two-sinusoid-input Describing Functions (PDF)
• Appendix E. Table of Random-input Describing Functions (PDF)
• Appendix F. Table of Sampled Describing Functions (PDF)
• Appendix G. Analytical Justification for the Filter Hypothesis (PDF)
• Appendix H. Introduction to Probability and Random Process (PDF)
• Index (PDF)