books.google.com - The landscape of homological algebra has evolved over the past half-century into a fundamental tool for the working mathematician. This book provides a unified account of homological algebra as it exists today. The historical connection with topology, regular local rings, and semi-simple Lie algebras...http://books.google.com/books/about/An_Introduction_to_Homological_Algebra.html?id=flm-dBXfZ_gC&q=natural+isomorphism&utm_source=gb-gplus-shareAn Introduction to Homological Algebra