This entry is about the book
Calculus of Fractions and Homotopy Theory,
Ergebnisse der Mathematik und ihrer Grenzgebiete, Band 35, Springer (1967)
This book first introduced a general localization of a category at an arbitrary class of morphisms (nowadays sometimes called the Gabriel-Zisman localization (see also Gabriel localization)); however the size issues are not discussed and the formalism of universes is understood as an excuse. The special case of categories of fractions is treated in detail.
The book has large historical importance for a clean and innovative formalism treating the interaction of category theory (including adjoint functors, Kan extensions, strict 2-categories), simplicial methods and homotopy theory. An important version of a definition of a homotopy category by the abstract categorical localization by the class of weak equivalences is introduced.
It has been proved that the homotopy categories of CW complexes and of simplicial sets are equivalent. The method of the notion of a fundamental category of a simplicial set (now sometimes also called the homotopy category), refining in a sense the notion of fundamental groupoid, is defined using adjointness.
The book is written in a recognizable abstract, clean and precise language with economic, rather short and formal, formulations.