Nearly any anthology of classic eerie fiction—from the old Modern Library Great Tales of Terror and the Supernatural to the compact New York Review Books paperback Shadows of Carcosa—will include Algernon Blackwood’s “The Willows.” Sometime early in the last century, two men are canoeing down the Danube when they stop to camp on a small island thick with willow trees. From the first, the narrator feels uneasy, as if they were interlopers. In fact, they are. The pair have stumbled upon the threshold to some other dimension or realm of being utterly alien to ours. Once the campers are detected by the island’s hostile entities, escape becomes impossible—unless there is a sacrifice.

To many, including H.P. Lovecraft, “The Willows” is the finest story in the canon of supernatural fiction. Nonetheless, it contains no ghouls or slasher-movie gruesomeness, or any clanking apparitions back from that bourn from which no traveler generally returns. Instead Blackwood instills disquiet, then apprehension through a series of increasingly inexplicable details—a glimpse of what seems to be an otter, something unnerving about the swaying of the trees, a peculiar gong-like humming, the appearance of funnel-like hollows in the sand. The story avoids explicitness, but instead tantalizes, draws us in, and frightens us with something not quite said. Little wonder that Blackwood deeply admired the artistry of Henry James’s The Turn of the Screw.

Blackwood himself is, arguably, the central figure in the British supernatural literature of the twentieth century. Vernon Lee (the penname of Violet Paget), Arthur Machen, and M.R. James all produced their best work as late Victorians. Lee’s Hauntings, highlighted by her seductive and unsettling “Amour Dure”—my own favorite ghost story—appeared in 1887; Machen’s major works, starting with The Great God Pan (1894), were products of fin-de-siècle London bohemia; and M.R. James’s Ghost Stories of an Antiquary, though published in 1904, comprised tales read aloud at Cambridge during Christmastime in the decade previous. James’s later collections maintain that same old-fashioned, port-and-stilton flavor.

All three of these writers are, in their differing ways, finer prose stylists than Blackwood—indeed, they rank among the finest in English—but none matches him in range, power, and sheer industry. Blackwood’s first magazine pieces, which began to appear in the early 1900s, launched a writing career that would eventually include some two hundred short stories and a dozen novels published over more than four decades. What’s more, he produced every kind of supernatural tale, from the sentimental to the horrific to the sacrilegious (in one, Jesus Christ appears at a dinner party), and of every length, whether the “ten-minute” short-short or the full-dress Edwardian bildungsroman. Blackwood’s most ambitious efforts, though, transcend the weird tale to become metaphysical fantasies. In them a deep affinity for nature or a heightened awareness of a…