- Oppenheim, Willsky, Nawab, Signals and Systems. The standard textbook. Mathematically careful, thorough, much-loved.
- B. P. Lathi, Signals, Systems, and Communications. A gentle, intuition-friendly introduction.
- Proakis, Digital Signal Processing. Covers Z-transforms and digital filter design in depth.
- Steven Smith, The Scientist and Engineer's Guide to Digital Signal Processing. Free online. The clearest practical introduction.
- Lyons, Understanding Digital Signal Processing. Practical, popular, with lots of plots.
- 3Blue1Brown, "But what is the Fourier Transform? A visual introduction" (YouTube). Excellent visual intuition for the Fourier integral as a "winding" of the signal around a circle.
section 10 of 101 min read