From Wikipedia:

Kripke delivered the John Locke Lectures in philosophy at Oxford in 1973. Titled Reference and Existence, they are in many respects a continuation of Naming and Necessity, and deal with the subjects of fictional names and perceptual error. They have never been published and the transcript is officially available only in a reading copy in the university library, which cannot be copied or cited without Kripke's permission. In fact many copies are informally circulated among philosophers. Its influence, though considerable, is thus difficult to trace.

I find it slightly odd that a philosopher can deliver a seven part series of public lectures and then control the transcript so that others are technically (if not actually) prohibited from even mentioning it in print. There's such a thing as speaking off the record, but surely this isn't it (for contrast, most of the John Locke lectures for the past decade and at least a large number of those before then have been published as books).