Jean Giraud 
Cohomologie non abelienne [PDF ebook] 

Now a common tool and object of study, stacks were introduced in this book as a framework for the study of non-abelian cohomology classes in degrees 1 and 2 on an arbitrary topos. The book shows that these cohomology classes can be represented by geometric objects called torsors and gerbes, and provides a detailed study of their basic properties. Since their introduction in this book, gerbes have become a widespread tool in geometry and topology.

A timeless classic, this French language book remains to this day a key reference for fibred categories, stacks, torsors, gerbes and topos.