“The monograph is self-contained … .” (Peeter Normak, zbMATH 1542.20003, 2024)“This book can serve as a well-written introduction for mastering methods of a well-developed theory and can be recommended to beginning researchers as well as to experienced researchers.” (Grigory I. Mashevitzky, Mathematical Reviews, August, 2024)“The monograph by Edmond W. H. Lee is a significant contribution to the research of semigroup varieties and varieties of semigroups equipped with additional unary or 0-ary operations. It will undoubtedly be interesting and useful to all researchers involved in the theory of varieties of semigroups, the theory of semigroups as a whole and related branches of general algebra.” (Boris M. Vernikov, Semigroup Forum, Vol. 107 (2), 2023)