"Fuzzy Equational Logic" is a specialized book that deals with similarity relations defined on a set with functions. These functions are designed to map similar elements to similar ones. The book covers fundamental mathematical properties of structures consisting of similarity-preserving functions, as well as logics used for reasoning about similarities. It provides a comprehensive and self-contained presentation of the concepts and results, supported by illustrative examples and graphical illustrations. This work is aimed at both researchers in the field and students who can use it as a graduate-level textbook. Its clear structure and accessible explanations make it a valuable resource for understanding fuzzy logic and similarity relations.