Handbook of Filter Synthesis, originally published in 1967 is the classic reference for continuous time filter design. The plots of filter behaviour for different designs, such as ripple and group delay, make this book invaluable. The discussion of how to synthesize a bandpass, bandpass, or bandstop filter from a lowpass prototype is also very useful.

Anatol I. Zverev is the author of Handbook of Filter Synthesis, published by Wiley.