Digital Filters

• Theory and Application of Digital Signal Processing, L. R. Rabiner and Bernard Gold Prentice-Hall 1975. This is a classic from Bell Labs and MIT that has stood the test of time and still valuable as a reference and introduction.
• Discrete Time Signal Processing, by A. V. Oppenheim and R. W. Schafer, Prentice-Hall, 2009. This seems to be a revision of the 1975 book by the same authors (some feat after 34 years) with a slightly different title (Digital Signal Processing). The older book was the definitive textbook on DSP.
• Digital Filter Design Handbook, by F. J. Taylor, CRC Press, 1983, out of print.
• Digital Signal Processing, by Roberts and Mullis, Addison-Wesley, 1987