Marx himself (according to Jappe) did not entirely grasp some of the fundamental forms of capitalism -- treating as essential characteristics what were actually traits of precapitalist forms of production. Thus, where Marx (in the 1860s) supposed that the working class would have to be excluded from the bourgeoisie and that the crisis of capitalism would arise from the shortcomings of the commodity form, by the 1960s it appeared that crisis had come instead from the total VICTORY of the commodity form and from the absorption of the proletariate (and his consciousness) INTO the bourgeoisie (and its commodified form).
Debord thus has the merit of having been one of the very first to understand that a Marxist form of analysis needed to be applied to very new and unexpected conditions. He was thus a transitional and seminal figure.
Part I contains an arid excursion into Marxist scholasticism, and lacks all of the brilliance and relentless intelligence of Plant's The Most Radical Gesture (hence, my ranking). Part II, which is the longest section, deals with Situationist practice, the events of May 1968 (which justified and vindicated their earlier pronouncements -- in which large segments of Parisian society simply went on a wildcat strike devoted... to play), and some brief, but beautiful tender obsevations on the Comment on the Society of the Spectacle. It is livelier.
On the question of the relation between Debord and Baudrillard, Jappe sides with Plant, and against the view that Debord was some type of cynical misanthrope, he argues that Debord was no nihilist, that he loved the Paris of the 1950s that he knew, and even the later traces of authenticity that could be found -- and that "[f]or the ostentatious despair that flirts with self-destruction and is so much admired in art galleries and halls of learning Debord had nothing but scorn: as early as 1955 he evoked the "overrated corpse" of Artaud....
This is not a brilliant book (like Plant's), but is sturdy and has utility.