He added to H.P. Lovecraft's Cthulhu Mythos cycle of stories with several works of his own, the most notable of which featured Titus Crow. Later works included the Necroscope series of novels, which produced spin-off series such as the Vampire World Trilogy, The Lost Years parts 1 and 2, and the E-Branch trilogy. There is also a follow-up anthology of short stories Harry Keogh and Other Weird Heroes.