Some comments/corrections on the first chapter, if you don't mind:
1. Theorem 2.4 is stated incorrectly. Given the context, I feel like this is worth correcting. Specifically, it says "degree n" rather than "degree at most n". Part of the proof purports to prove that the degree is indeed n but of course it doesn't because that needn't be true.
There are other cases where you say "degree n" for "degree at most n". Again usually this would be a minor error not worth pointing out, but in this context it seems worth getting right.
2. At one point you introduce a convention that deg(0)=-1. Later, in the exercises, you ask, is this really such a good convention? (The answer being, of course, no.) IMO you should anticipate this. Indeed I don't think you should state, as you do, "By convention the zero polynomial is defined to have degree -1", because that suggests it's some standard universal convention, which is definitely correct, and it's neither of those. Rather you should say something like "We'll use the convention that the zero polynomial is defined to have degree -1". But anyway, the point I made is that, if you're going to question its correctness later, you should anticipate that here, maybe saying something like "(Think about whether this convention makes sense.)" Or maybe not, and just getting rid of the absolutism of your current wording is sufficient. Either way, getting rid of that absolutism and certainty is good; you want to encourage to people about this sort of thing immediately, not encourage them not to think about it until later.
3. You say that when you see a definition you should write down examples. I would add, "and non-examples". Ideally non-examples that come as close as possible but don't quite make it. You touch on this a little with your polynomial examples, but it's worth stating explicitly.
(In some cases non-examples are unnecessary, but in the generic case one should look for them.)
4. Regarding your polynomial examples, you don't justify that they are, in fact, not polynomials. Now of course you don't, that would be too hard to do here and take up lots of space you want to use for other things. That's fine. But if you're not going to do it, you should call out that you're skipping over it, like you do with other things. After all, all sorts of nonobvious things can be polynomials -- such as (x-1)(x+6)^2, as you pointed out earlier, but included no similar examples here. (Yes that's obvious to anyone who knows anything about polynomials, but my point is that it's not in the correct syntactic form.) Like, x^e - x^e is a polynomial, you know? Because it's 0. So without some more knowledge, you can't immediately conclude that your example x + x^2 - x^pi + x^e is in fact not a polynomial! You should make a note of that, as I said.
5. I feel like it's likely worth noting somewhere in this chapter that actually in general in math it's the "syntactic" definition of polynomial that turns out to be the right one (you don't want to define polynomials to be functions if you're working over a finite field, say!). Maybe not and that would just be confusing, I dunno.
6. This is just nitpicking, but I'd suggest rewriting Theorem 2.3 in a clearer, more standard way. "A nonzero polynomial of degree n has at most n distinct roots." What you wrote down is equivalent, of course, but (IMO) harder to read.
Otherwise, this is pretty nice. I remember being distinctly confused by stuff like "the product over j not equal to i" when I was a kid. I imagine it'll be quite helpful to a number of people that you're laying things out like that explicitly.
Actually, sorry, on that note, one further comment:
7. You comment on how sigma and pi notation are special cases of fold, but you might want to make a further note about how (unlike general folds) these are folds where the order doesn't matter, and that the fact that the order doesn't matter is one of the things that allows notation like "product over j not equal to i".