[quote author="gyraf"]Duncan's "Active Filter Cookbook"[/quote]
Do you possibly mean Don Lancaster's AFC gyraf?
Adam, you need to give some guidelines for the level you seek. The literature on filters is amazingly vast, and ranges from "cookbooks" to highly abstruse treatises.
One significant gap, however, is in the very area of audio applications: for equalization we want shelving filters and peaking/dipping filters, and although these are within the scope of the literature on generalized biquadratic and lower/higher-order transfer functions, they are not usually described in much useful detail. Instead we have no end of highpass, lowpass, bandpass, and notch filters of various shapes and sizes. We need these, definitely, but for EQ we usually want low-to-moderate sharpness transfer functions whose magnitude never goes to zero---say a dip that is 5dB deep and with a fractional halfwidth of 2/3 of the center frequency. It's hard to find this stuff---enough so that some companies treat it as proprietary.
Rane I suspect has published some pretty good material, although I haven't looked at much of it. Search their site or drop Dennis Bohn a note (I assume he is still there when I say that).
Recent literature in JAES has tended to focus on digital filter implementations of audio-style filters, so may not be too helpful. I recall a paper by Shpak (think that's the spelling) on peaking/dipping filters from a few years ago along those lines, and it may have had some references to analog filter art.
For passive filters the generally acknowledged "bible" is Zverev's Handbook of Filter Synthesis, still in print after many years and which I notice is now in paperback.
