I hate to throw shade on this, but look at theorem 2.1, proving by induction that x*n - 1 is divisible by x-1. The concluding sentence I believe mistakenly refers to the left hand term, when they intend to refer to the right hand term.

"But the left term is divisible by x - 1 by the induction hypothesis, and the right term is just x-1.".

I would spend many hours staring at that sentence wondering why I "just" did not understand, cycling through a whole range of feelings, years later to find out that there was likely a typographical error.

Additionally, many textbooks like this give a complicated expression and rather than reduce it mathematically, they expect the reader just to do this in their head and come to the conclusion that it is true.

Such things may seem like trivialities, and perhaps I am not the smartest math person around, but not paying attention to these subtle points can lead to deep frustration for the reader.