Here’s a list of new books from dead authors, including an Umberto Eco essay collection taken from L’Espresso magazine, Chronicles of a Liquid Society. Eco “sees with fresh eyes the upheaval in ideological values, the crises in politics, and the unbridled individualism that have become the backdrop of our lives—a ‘liquid’ society in which it’s not easy to find a polestar, though stars and starlets abound.”
Also, Sleep No More: Six Murderous Tales by P.D. James. These are not unpublished stories, but stories written as far back as 1973 that have never been collected.
C.S. Lewis had an impressive posthumous output, thanks in part to the fact that Walter Hooper saved several boxes of his papers from the bonfire.