Perfect monoids

Banaschewski, B., Projective covers in categories of topological spaces and algebras, Proc. Conf. on Topology (Kanpur, 1968).

Banaschewski, B., Injectivity and essential extensions in equational classes of algebras, Proc. Conf. on Universal Algebra (Kingston, 1969), pp. 131–147.

Bass, H., Finitistic dimension and a homological generalization of semi-primary rings, Trans. Amer. Math. Soc.95 (1960), 466–488.

Berthiaume, P., The injective envelope of S-sets, Canadian Math. Bull.10 (1967), 261–274.

Clifford, A. H., and Preston, G. B., Algebraic Theory of Semigroups, Vol. II, Providence, 1967, Chapter 11.

Eckmann, B. and Schopf, A., Uber injektive Moduln, Arch. Math. 4 (1953), 75–78.

Hofmann, K. H., and Mostert, P., Elements of Compact Semigroups, Columbus, 1966.