Theory of the Avant-Garde is probably the best book on aesthetics I've ever read. Burger argues that aesthetic theory has previously neglected to deal with art as an institution in bourgeois society. Unlike medieval works, art under capitalism is relatively autonomous from various social institutions. While sacral art was integrated into the church, and courtly art into the court, art under capitalism develops into its own sphere, to be judged exclusively by aesthetic criteria. At the same time, art is separate from the praxis of life; its value consists precisely in its ability to take one away from everyday experience. Drawing from Marcuse's classic essay "On the Affirmative Character of Culture," Burger argues that art's aesthetic unity combined with its autonomy from the praxis of life serve to negate the political engagement of any individual work. This palliative effect in turn exposes the partiality of autonomous art; its autonomy actually serves a deep structural function of a cultural anodyne.
The avant-garde comes in as the first moment that artists became self-conscious of the effects of art's autonomy and sought to destroy it. Burger uses a Hegelian-Marxist framework (although he is sensitive to teh Althusserian critique) to argue that the avant-garde constituted the moment of self-criticism of the institution of art which revealed the truth of the institution's past, just as Marx's self-criticism of bourgeois society produced historical materialism, making possible a generalized science of history.
This summary only scratches the surface of Burger's 100 page essay. In developing this central argument, he has fascinating arguments regarding Benjamin, Adorno, Lukacs and Brecht, and thought-provoking readings of various avant-gardiste techniques. I'd venture to say there are few books triple the length of this one that contain the same amount of careful, creative, and exciting writing.