An introductory abstract algebra textbook might not be too bad.
I found the topic fascinating, but very hard work because I had to discard so many mental assumptions. Its difficulty lies within, in abandoning your intuition. Once your intuition has been sufficiently expanded, it probably looks easy. (Though my recollection, dim after almost five decades, still shudders at the word "kernel.")
And of course it can branch out in many other ways, for example adding associativity first gives what's called a semigroup.
My dim recollection is that we spent a fair amount of time on semigroups, which my teacher said had enough structure to be interesting but not so much as to be confining.
It is far and large my favorite subject, but I'm starting to think my brain is not built for it :cool: I enjoyed the course (despite the work), but it convinced me that I was no mathematician. I could only follow where others had blazed a path.