I would like to see references out to less hand wavy material. It’s a free resource, so I’m not going to complain too hard. Gnarly windowing effects on the square wave example in the first chapter; I do think if you’re going to show something like that, you should show that it’s dependent on sample rate and duration. Maybe that’s coming later in the text. To be fair, the perfect train of sinusoids you get in a linear systems class is equally unrealistic.