This site purports to give you the answer, though I've no idea how accurate it is. For example, it reports that for "Brian Leiter," there are "16 works in 25 publications in 2 languages and 2,403 library holdings; though they seem to have missed the Chinese translation of one book, and the library holdings they report for some books seem lower than what sales figures would lead one to believe. (They report that 278 libraries hold copies of my Nietzsche on Morality [2002], which has sold about 5,000 copies, which would suggest that the overwhelming majority of the sales were to individuals, rather than institutions, which I guess is possible, but seems to me not likely.) By contrast, my colleague Martha Nussbaum is recorded as having "171 works in 254 publications in 12 languages and 25,584 library holdings," which, relative to my results, sounds like it could be right!
UPDATE: Dean Rowan, a librarian at Berkeley, writes:
You are correct to wonder why few occurrences of your Routledge Nietzsche book appear in WorldCat when you know it has sold several thousand copies. Oddly, the 278 figure becomes 320 if you view the full record. I can't explain that discrepancy, but I can explain at least in part why the number of holding libraries is fewer than what you expect. Although OCLC (WorldCat) aggregates the holdings of some 10,000 or so libraries, it is not an exhaustive "union catalog." For quite some time, academic libraries tended not to include their holdings in OCLC, opting instead for alternative services. Recently, one of those major services, RLIN, was purchased by OCLC. I'm not sure whether that means that holdings of academic libraries on RLIN would automatically be transferred to OCLC. Our library at Boalt has indeed migrated, but lots of libraries keep their distance from OCLC. This must certainly account for some of the discrepancy, although I would not have expected it to remain so large. That is, I would have imagined that many more academic libraries on RLIN had elected to migrate. But if OCLC can't square 278 with 320, there must be other hiccups at work.
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