Digging deeper into the HN thread that you linked to, I came across this discussion [1] on Reddit which is focused more on the abstract algebra question. They r

Digging deeper into the HN thread that you linked to, I came across this discussion [1] on Reddit which is focused more on the abstract algebra question. They recommend Algebra: Chapter 0 by Aluffi (Amazon [2] or PDF [3]). [1] http://www.reddit.com/r/math/comments/1eiyid/category_theory_prereqs_does_alg_geometry_come/ http://www.reddit.com/r/math/comments/1eiyid/category_theory... [2] http://www.amazon.com/Algebra-Chapter-Graduate-Studies-Mathematics/dp/0821847813?tag=duckduckgo-d-20 http://www.amazon.com/Algebra-Chapter-Graduate-Studies-Mathe... [3] http://www.mimuw.edu.pl/~jarekw/pdf/Algebra0TextboookAluffi.pdf http://www.mimuw.edu.pl/~jarekw/pdf/Algebra0TextboookAluffi....