Horkheimer and Adorno basically took over the first, marxist Frankfurt School and turned it into something very different, even though they thought they were Marxists and were much influenced by Lukacs (Geschichte und Klassenbewußtsein). Benjamin was a friend of Adorno's, but stayed basically on the outside of the Frankfurt school thing, although after 1933 he (and others) were very dependent on the Adorno/Horkheimer money and Beziehungen and such and had to dance to their tune.
Adorno also was a romantic philosopher, a fierce critic of Hegel, but from the inside of the German idealistic, but non-Kantian and Hegelian ("dialectic"), tradition; and he was thought he was a kind of dialectic thinker, even though he could not stand Hegel's branch of dialectic logic with its (Adorno thought) smug synthese machine. Adorno wanted to have an open-end dialectics without "prefabricated" results. He wanted to think and to have something completely different, the non-identical, and really new.
Adorno published an interesting and not too difficult book on Doing Philosophy, the Philosophische Terminologie lectures. Also famous is his essays about the Whole being not Truth, as Hegel had wanted, but Untruth (Das Ganze ist das Unwahre or something like that). His best book is perhaps the Minima Moralia (reflections), but his philosophical opus magnum probably the massive Negative Dialektik. There also was an Ästhetische Theorie.--Ralf Heinritz 18:52, 9 March 2010 (UTC)