To be a bit more concrete than the grandparent's name soup, I liked this handbook article about unification in theories: Franz Baader and Jörg Siekmann. Unification Theory. In D.M. Gabbay, C.J. Hogger, and J.A. Robinson, editors, Handbook of Logic in Artificial Intelligence and Logic Programming, pages 41-125. Oxford University Press, Oxford, UK, 1994.
This seems to be a newer version with similar contents but different authors: http://www.cs.bu.edu/~snyder/publications/UnifChapter.pdf
This is specific to unification and not to broader equational reasoning.