Resources for learning about filters from scratch?
The best book I know that exclusively treats digital filters is Digital Filter Design by Parks and Burrus.
But any good textbook on signal processing has one or more chapters about properties and design of digital filters. And since the most common IIR filter designs are based on analog filters, these books usually also treat the classic analog filter approximations like Butterworth, Chebyshev, Cauer (elliptic), and Bessel.
Two good undergraduate texts are Digital Signal Processing by Proakis and Manolakis and Digital Signal Processing Using Matlab by Ingle and Proakis.
Also take a look at the answers to this very related question.