Actually, it's not that hard. The difficulty lies in too many concepts. You have to remember and understand slowly. Everything you prove is basically based on concepts. The real difficulty lies in proof skills, but beginners basically focus on mastering concepts. This course is closely related to number theory, and a considerable part of knowledge needs to use the conclusions in number theory.

Recommended by Nie Ling's female horse. Ding. Introduction to Algebra (2nd Edition). Beijing: Higher Education Press, 2000.

This book is very detailed, and other books are basically superficial or messy.