The British novelist, student of arcane lore, and mythographer John Cowper Powys (1872–1963) is known today by only one or two of his novels — “Wolf Solent” (1929), and perhaps “A Glastonbury Romance” (1932). These are vast, multilayered works whose heady mixture of Celtic mysticism and sexual psychology has won them a coterie readership. Those who have wondered what sort of personality lay behind such peculiar epics can now find out from Morine Krissdóttir’s densely detailed “Descents of Memory: The Life of John Cowper Powys” (Overlook Press, 480 pages, $40), based largely on unpublished materials, and can read a late work, “Porius” (Overlook Press, 751 pages, $37.95), in its full text for the first time. Whether either book will win new converts for Powys, however, is open to question.

The biography reveals an outlandish figure. Born into a clerical family, the eldest of 11 children, Powys began psychoanalyzing himself from an early age. He was raised to regard sexual desire as sinful, and his writing became a way of exorcising inner demons. The symbol of the labyrinth is a recurring one in his work: He saw himself as both Theseus, the rational man finding his way through the maze, and the Minotaur, the bestial creature which must be hidden from human view. Mundane day-to-day existence was less vivid to him than this inner life of the mind, and he became convinced that he was a bard-magician, in touch with supernatural forces: He was convinced that he was communicating with the spirits of dead Indians behind Phudd Bottom, his cottage in upstate New York, and he regularly gave magical names to local landmarks, such as trees or stones, in order to control them. This animistic response to nature lies behind his best neo-mythological prose, but when Powys’s control falters, the result is often incoherent.

“Porius” is a telling example. Set in October, 499, in a Roman fort in North Wales, it depicts the power struggle between the Celts, who rule the Roman client kingdom of Endymion, and the Saxons, who hope to conquer it. King Arthur and Merlin lend aid to the Celtic Prince Porius, and the Saxons are defeated. Powys’s central interest, however, seems to be Porius’s erotic and military exploits, which he examines with microscopic care. Though the action of the novel spans a single week, many readers will not last beyond Thursday. The problem is that too many things of unequal interest are happening at once, all of them described in an exhaustingly baroque style that assumes we know as much about arcane matters as the author, and that we share his volcanic energy. If only we did.

Given Powys’s hermetic cast of mind and solitary temperament, formal education made little impact on him. Bullied at Sherborne, his public school, and lacking a wide social circle at Cambridge, he remained in many ways a child to the end. He diagnosed himself as an “anti-Narcissist,” despising his own physicality, and also as an escapist: “My whole life has been one long running-away,” he wrote in 1916, “and the years have given me swiftness and agility.”

Powys found work as an itinerant lecturer, first in England, and then for 25 years in America, where he met the much younger Phyllis Plater, daughter of a Missouri businessman. Though he was already married to the daughter of a close friend, she became his companion from 1923 until his death, and their relationship was concealed from his wife and son. The most ground-breaking work in Krissdóttir’s biography is its fullness of detail about Phyllis, who had been a shadowy figure in previous accounts. She burned all her letters to Powys in 1951, but recently some early correspondence and diaries have surfaced, of which Krissdóttir makes extensive use. Powys nicknamed Phyllis “the T.T.,” short for “Tiny Twig” (“Tylwyth Teg” in Welsh), casting her as a fairy, his elemental attendant. She was also the target of less agreeable fantasies, some of them sadomasochistic.

Powys was a spellbinding platform orator — at the end of one two-hour discourse on Hardy, a crowd of two thousand people rose to its feet, roaring for more — but as radio and cinema flourished in the 1920s, the demand for public lectures dwindled, and it became increasingly important for Powys to support himself by writing. Thus he abandoned a vocation he loved, and at which he was conspicuously successful, because fiction paid better — an unusual turn of events indeed.

Powys had already published some apprentice novels, but “Wolf Solent” (1929) truly launched his career. It established the Powys manner: the simultaneous presence, on different but interacting planes, of the everyday and the paranormal, the elliptical allusions, the panoramic sweep, above all the rhapsodic style which takes the gamble of demanding our total, uncritical surrender, and fails if we resist. Critics at the time would have recognized in this the same resonances they had found in D. H. Lawrence, and something of the same intensity of realization.

Powys read Jung, and imagined himself a visionary tapping primordial levels of consciousness. He invented his own private rituals, prayers, and holy places. He wrote a psychoanalytic autobiography in which reliable facts are hard to come by. He turned out self-help manuals whose message Powys privately summed up as “Be Happy Damn It!” Money was desperately short.

In 1935, when he was 63, he and Phyllis left America for Wales, where he remained for the rest of his life, devolving first into increasingly obtuse literature of alchemy, and finally into embarrassing sexual infantilism and voyeurism.

His infantile streak dominated him, and Phyllis was now part-nurse, part-mother surrogate. His diet consisted of milk, tea, stale bread, and olive oil. For the last three years of his life, he remained indoors, surrounded by photographs of his siblings, past and present merging in his mind as the sun’s rays melt into twilight.

Powys often reminds us of other writers, but the similarity is never exact. Like Blake, he inhabits an unseen world, but Blake was also an intensely practical man. Like Hardy, he is a writer of the west of England, but his pessimism is never as unrelenting as Hardy’s. Like Lawrence, he was a lyrical, sometimes lush, writer about sex, but Lawrence can also be seen as a novelist of realism, and could regard his own obsessions more objectively. Powys’s depiction of the processes of human consciousness aligns him with other experimental writers of the 1920s. Ultimately, however, in his amalgamation of ancient myth and modern psychology, which never quite coheres, Powys stands alone, the only believer in a religion he had invented himself.

Mr. Dean is head of English at Dragon School, Oxford, and a fellow of the English Association.