Les Dérivateurs is a manuscript of some 2000 pages left behind by Alexander Grothendieck when he disappeared in 1991, dealing with the fundamentals of homotopy theory. It can be considered as a sequel to the more famous Pursuing Stacks, but has a less conversational style.
Portions have been LaTeXed and are available here. More accessible are Maltsiniotis’ introductory notes (Postscript).