There was some chatter on Twitter about this, but here's what Google Scholar tells us about which books are being cited the most.
Martha Nussbaum (Chicago) looks to be the hands-down winner, with multiple books cited more than 1,500 times, including Women and Human Development (2001), with nearly 11,00 citations; but also Frontiers of Justice (2009), with 6,100 citations; Upheavals of Thought (2002), with 6,600 citations; and Hiding from Humanity: Disgust, Shame and the Law (2009), with more than 2,000 citations.
Here are some other philosophy books published since 2000 with more than 1,500 citations according to Google Scholar (cite count rounded to the nearest 100):
Timothy Williamson (Oxford), Knowledge and Its Limits (2000), 5,400 citations
Miranda Fricker (CUNY Grad Center), Epistemic Injustice (2007), 4,000 citations
Cristina Bicchieri (Penn), The Grammar of Society (2006), 2700 citations
Jerry Fodor (late of Rutgers), The Mind Doesn't Work That Way (2001), 2,600 citations
Robert Brandom (Pittsburgh), Articulating Reasons (2009), 2,300 citations
Ted Sider (Rutgers), Four-Dimensionalism (2001), 2,200 citations
Ronald Dworkin (late of NYU), Justice for Hedgehogs (2011), 2,100 citations
Timothy Williamson (Oxford), The Philosophy of Philosophy (2008), 2,000 citations
Derek Parfit (late of Oxford), On What Matters (2011), 1,900 citations
John Hawthorne (ACU & USC), Knowledge and Lotteries (2004), 1,800 citations
Philip Kitcher (Columbia), Science, Truth and Democracy (2003), 1,800 citations
Christine Korsgaard (Harvard), Self-Constitution (2009), 1,700 citations
Brian Skyrms (UC Irvine), Evolution of the Social Contract (2014), 1,700 citations
Jeff McMahan (Oxford), The Ethics of Killing (2002), 1,600 citations
Brian Skyrms (UC Irvine), The Stag Hunt and the Evolution of Social Structure (2004), 1,600 citations
Ernest Sosa (Rutgers), A Virtue Epistemology (2007), 1,600 citations
Jaegwon Kim (late of Brown), Physicalism, or Something Near Enough (2007), 1,500 citations
Obviously the more recently a book was published, the harder it is to rack up 1,500 or more citations. So here are some well-known works from the last decade that seem headed to citation counts like those above:
Christian List & Philip Pettit, Group Agency (2011), 1200 citations
Philip Pettit, On the People's Terms: A Republican Theory and Model of Democracy (2012), 1,000 citations
John MacFarlane, Assessment Sensitivity (2014), 800 citations.
But in that most important sub-field of philosophy, Nietzsche studies, no one in the last twenty years can match by Nietzsche on Morality (2002) with nearly 900 citations!
Comments are open for readers to add philosophy books with more than 1500 citations since 2000, or with more than 750 citations since 2010.