Julius Smith @ Stanford has a couple of great free books online: Introduction to Digital Filters with Audio Applications https://ccrma.stanford.edu/~jos/filte

Julius Smith @ Stanford has a couple of great free books online: Introduction to Digital Filters with Audio Applications https://ccrma.stanford.edu/~jos/filters/ https://ccrma.stanford.edu/~jos/filters/ Mathematics of the Discrete Fourier Transform https://ccrma.stanford.edu/~jos/mdft/ https://ccrma.stanford.edu/~jos/mdft/