Deleuze is perhaps the most authoritative of Foucault's commentators. Not because of their friendship, collaborations, or dialogues. Not even because of the enormous respect each held for the other. If this book were written by a stranger, it would still be a part of the Foucault canon. Because it grasps hold of the Foucault project and grapples with it, refusing to let go and refusing to let it slink away.
The book is divided into two, where the first part includes articles previously published on certain works (The Archeology of Knowledge, and Discipline and Punish), and the second struggles to elucidate and elaborate the project of "Thinking Otherwise". This part embraces both Foucault's early thoughts on transgression and his later accounts on techniques of the self. It draws on his histories and his theories of power: Foucault's method and his objectives. It also reflects Deleuze's own concerns with thought and the fold.
In terms of texts, this book is important because it expands on the thoughts only sketchily worked out in his article in "Magazine Litteraire" (published 1994: written in 1977, and translated in Davidson's Foucault and his Interlocutors, UofChicago Press, 1997). It is also tantalising in its hints towards and references to Foucault's unpublished work in volume 4 of the History of Sexuality.