By our count, Mary Oliver has published 28 books. About 22 of those are books of poetry (it's a little hard to count, since a few books are a mix of poems and essays). We're impressed.
Mary Oliver attended both Ohio State University and Vassar College in the 1950s, but did not end up getting a degree. Don't get any ideas! Unless, of course, you've also got a Pulitzer Prize-winning book in you. (Source)