I believe that a worthwhile book can be represented in three quick ways. A worthy book is written only if (1) you think that the books on the same or a related subject fall into a sort of general error (polemical function of a book); (2) you think that something essential about the subject has been forgotten (inventive function); (3) you consider that you are capable of creating a new concept (creative function). Of course, that’s the quantitative minimum: an error, an oversight, a concept. … Henceforth, for each of my books, abandoning necessary modesty, (will ask myself (1) which error it claims to correct, (2) which oversight it wants to repair, and (3) what new concept it has created.
Gilles Deleuze, Letter to Arnaud Villani. December 29, 1986. Cited in Gille sDelueze & Félix Guattari: Intersecting Lives, by François Dosse. Translated by Deborah Glassman. New York: Columbia UP, 2010.