UPDATE (12/16): I'm moving this to the front, in hope of eliciting more comments (thanks to those who have posted so far). More than 1,000 visits to the site yesterday, and while many are law students and faculty, I'm sure there were more philosophers than the 15 who posted. Don't be bashful!
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I thought it might be interesting to invite philosophy faculty and graduate students (and those in cognate fields with significant philosophical interests) to list (and, if you like, comment on) what you take to be the most significant books over roughly the last quarter-century in your field. "Significant" books need not be books you agree with, just ones that are of high quality and have been an important stimulus to additional work. I realize, of course, that articles are often as significant as books in philosophy, and so collections of papers are welcome too. Individuate the sub-fields of philosophy however it makes sense to you to do so. Please don't post anonymously and say who you are: e.g., "Brian Leiter, Faculty, UT Austin." Also, try not to list more than three books in each field.
Hopefully, the resulting list will provide some useful guidance (to me and other readers) as to what books deserve attention for those not working in that immediate area.
Here's my list:
PHILOSOPHY OF LAW:
Joseph Raz, The Authority of Law (Oxford, 1979)
John Finnis, Natural Law and Natural Rights (Oxford, 1980)
Leslie Green, The Authority of the State (Oxford, 1988)
METAETHICS:
Allan Gibbard, Wise Choices, Apt Feelings (Harvard, 1990)
Michael Smith, The Moral Problem (Blackwell, 1994)
Peter Railton, Facts, Values and Norms (Cambridge, 2003)
NIETZSCHE STUDIES
Maudemarie Clark, Nietzsche on Truth and Philosophy (Cambridge, 1990)
Peter Poellner, Nietzsche and Metaphysics (Oxford, 1995)
John Richardson, Nietzsche's System (Oxford, 1996)
MARX/MARXISM
G.A. Cohen, Karl Marx's Theory of History (Princeton, 1978)
Raymond Geuss, The Idea of a Critical Theory (Cambridge, 1981)
Michael Rosen, On Voluntary Servitude: False Consciousness and the Theory of Ideology (Harvard, 1996)
FREUD/PSYCHOANALYSIS
John Deigh, The Sources of Moral Agency: Essays on Moral Psychology and Freudian Theory (Cambridge, 1996)
Sebastian Gardner, Irrationality and the Philosophy of Psychoanalysis (Cambridge, 1993)
Jonathan Lear, Open-Minded: Working out the Logic of the Soul (Harvard, 1998)