I was asked thoughtful question: why *not* read Oppenheim/Schaffer? Long-winded answer: we teach DSP the same *wrong* way we teach calculus: remember limits→derivatives→integrals? Well, Newton &co did integrals, then Cauchy &co did derivatives, and only then did Weierstrass &co nail down limits. High school calc reverses the history order and the usefulness order because it turns out that for integrals and derivatives to be rigorous, you need limits. This took us centuries to understand.