I am not really complaining about Discrete Math. The book listed in Intermediate is fairly/horrendously difficult for someone who hasn't seen elementary treatment of the subject before.
Also, there are newer books of comparable quality to old classics like Algebra by Birkhoff/Maclane. If nothing, they have updated prose. Chapter 0 by Aluffi is phenomenal, for example.
I think this list could use some updating. Most books on the list are super old. Also, from cursory look, there appears to be only one book on discrete math in Intermediate. There are tons of discrete math books that should serve as stepping stones for the one listed. For example, Discrete Intro to Math by Edward Scheinerman and Discrete Math by Susanna Epp.
Being able to use something and knowing the whys are two different things. Many people know Calculus(practice), but not many know Real Analysis(theory behind Calculus, the academical bit).
"In mathematics, the quaternions are a number system that extends the complex numbers."
Naturals are a subset of integers. Integers are a subset of rationals. Rational are a subset of reals. Reals are a subset of complex numbers. The latter are a subset of quaternions. It's not difficult to construct each number system. If you look inside any undergrad math textbook, you'll find exact same writing style showcased in the Wikipedia article on quaternions.