I would like to mention the little known (but excellent) book by W. Wechler, Universal Algebra for Computer Scientists, Springer- Verlag, Berlin (1992).
SBN: 978-3-642-76773-9 (Print) 978-3-642-76771-5 (Online).
It covers equational theories in great detail but also treats topics that are hardly found elsewhere, like multi-sorted algebras or ordered algebras.
Contents of the book.
1 Preliminaries
Basic Notions (Sets, Algebras
Generation, Structural Induction, Algebraic Recursion and Deductive Systems)
Relations (Regular Operations, Equivalence Relations, Partial Orders,
Terminating Relations, Well-quasi-Orders, Cofinality, Multiset Ordering
and Polynomial Ordering).
Trees (Trees and Well-Founded Partially Ordered Sets, Labelled Trees,
$\omega$-Complete Posets and Fixpoint Theorem, Free $\omega$-Completion).
2 Reductions
Word Problem (Confluence Method, Word Problem for Congruences)
Reduction Systems (Abstract Reduction Systems, Term Rewriting Systems, Termination).
3 Universal Algebra
Basic Constructions (Subalgebras and Generation, Images and Presentation, Direct Products
and Subdirect Decompositions, Reduced Products and Ultraproducts).
Equationally Defined Classes of Algebras (Equations, Free Algebras,
Varieties, Equational Theories, Term Rewriting as an Algorithmic Tool for
Equational Theories).
Implicationally Defined Classes of Algebras (Implications, Finitary Implications
and Universal Horn Clauses, Sur-Reflections, Sur-Reflective Classes,
Semivarieties and Quasivarieties).
Implicational Theories, Universal Horn Theories, Conditional Equational
Theories and Conditional Term Rewriting.
4 Applications
Algebraic Specification of Abstract Data Types (Many-Sorted Algebras, Initial
Semantics of Equational Specifications, Operational Semantics).
Algebraic Semantics of Recursive Program Schemes (Ordered Algebras, Strict
Ordered Algebras, w-Complete Ordered Algebras, Recursive Program Schemes.
Appendix 1 : Sets and Classes.
Appendix 2 : Ordered Algebras as First-Order Structures.