Theodor W. Adorno (1903–1969) was a first-generation member of the Frankfurt School For Social Research. As was the case with his fellow 'Critical Theorists' Horkheimer and Marcuse et al, his work was interdisciplinary and considerably Marxist. His task, as he saw it, was to use philosophy, sociology and other tools to understand the evils of the past and present, thereby helping to prepare the possibility – he thought one could do little more than that – of a better future. Adorno’s largest philosophical debts are to Kant, Hegel, Marx and Nietzsche. Adorno drew upon those thinkers, and others including Freud and Walter Benjamin, in creating a distinctive approach to metaphysics, epistemology, ethics, aesthetics, and political philosophy.
Adorno’s magnum opus, Negative Dialectics (first published 1966), is as much a critique of metaphysics, epistemology and of systematic philosophy in general as it is a contribution to those endeavours. Somewhat similar – and working via an account of Husserl’s phenomenology – is the earlier Metacritique of Epistemology (first published in 1956). The posthumously published Aesthetic Theory is also very important; given the centrality of aesthetics to Adorno’s philosophy, it is not ‘just’ a work on aesthetics. There are other important works. Dialectic of Enlightenment (co-written with Max Horkheimer; first published in 1947) presents a case that the Enlightenment has disastrously misfired. Minima Moralia (1951) is a careful and quintessentially Adornian aphoristic text, covering many subjects but especially social, political and ethical ones. Hegel: Three Studies (1963) is something of a preparation for Negative Dialectics. Among the recently translated lecture courses, Problems of Moral Philosophy is perhaps the most enlightening.
Theodor Adorno: Key Concepts, edited by D. Cook, might be the most accessible introduction to this difficult thinker. Other introductions include works by B. O’Connor, G. Rose, S. Jarvis, and chapters 5 and 7 of D. Held. Adorno’s lectures (such as the aforementioned Problems of Moral Philosophy) are also useful as introductions.