The two books that I am thinking of seem to be out of print currently. There's a gossipy book by Peyser, that has third hand references to it, but the book I'm thinking of (and I have it in my books in storage) has documented diary references to a specific incident in the late 1920s.

Hmm. I may need to check out the Gershwin collection at the Library of Congress to find the references myself.