An anthology projected to be the third book in the influential Dangerous Visions series edited by Harlan Ellison. Dangerous Visions was published in 1967 and was an anthology of stories with themes and/or content which was "dangerous" by contemporary sf standards. It included major introductions by Ellison, alone probably worth the price of the book. It was followed in 1972 by Again, Dangerous Visions, with LDV a projected third volume.
LDV would have been massive — an announced contents list from 1979 ran to 113 stories — and along with Ellison's traditionally enormous introductions (probably never written) may well have been unpublishable. In any event, it was never published and became a bone of contention between Ellison and nearly everyone else. By the late 70s, phrases like "When LDV is published" had become approximately equivalent to "When pigs fly," particularly since Ellison continued to insist that it would be published.
Another critique of LDV was that the stories in it might have been "dangerous" when commissioned in the early 70s, but had been left behind by the evolution of the field (in some part pushed by Dangerous Visions itself) and had been turned by time into curiosities or even embarrassments.
Christopher Priest published a critique of the affair in the late 80s as Last Deadloss Visions, which was published in a new edition in 1994 as The Book on the Edge of Forever (a play on the title of Ellison's famous Star Trek episode "City of the Edge of Forever"). Ellison was not at all pleased.