Followers of the work of Frank Brickle and I know that Frank has for years followed and promoted the work of Julius O. Smith. I subscribe to this support. Fortunately, JOS has done a lot of publishing his work online for years. And he is winnowing down into books such as
Introduction to Digital Filters with Audio Applications (at Amazon)
Introduction to Digital Filters with Audio Applications (at Barnes and Noble)
as well as his awesome treatment of Mathematics of Discrete Fourier Transforms belongs on the signal processor's shelf.
I have also looked through quite a few other books and have purchased these and wish to recommend them:
Introduction to Digital Signal Processing Filter Design by B.A. Shenoi
Digital Filters Basics and Design by Dietrich Schlichthärle
With these books on my shelf, I am able to make very good progress on my longer term goals of rending "fdatool for free as in beer" for Octave and SciPy.