Forgotten authors No 10: Gladys Mitchell

Born in 1901, she was one of the "Big Three" female mystery novelists, judged the equal of Dorothy L Sayers and Agatha Christie, but that's not quite accurate – she's more like a mad combination of both. Philip Larkin loved her and many admired her mordant and morbid mysteries. Diana Rigg starred in some bland TV versions of her novels that turn her Mrs Bradley character into a glamorous Miss Marple. The exposure has resulted in new demand for her books, which is good because they're more interesting than Christie's, if more problematic.

Virago republished The Rising of the Moon, but there are some 66 other volumes. Mitchell's old lady detective has little of Miss Marple's cosiness. Thrice married, she's physically repulsive, parchment-skinned and usually likened to a witch, a vulture or even a pterodactyl. In Dead Man's Morris she's described as having "the maternal anxiety of a boa-constrictor which watches its young attempting to devour their first donkey".

Mitchell was a schoolteacher who believed in the idea of the professional, progressive and somewhat Sapphic woman. Her title character was controversial and emancipated, and even considered murder justifiable if the occasion demanded. With such an outspoken heroine, Mitchell naturally made enemies. The Spectator described her as a "tiresome old trout" whose mannerisms were the most trying in detective fiction, but many adored her work. Her murder cases have ambiguous solutions, and an air of the supernatural is never entirely banished from them. Her plots are on the farthest side of credulity, but to worry about realism is to miss the fun of her storytelling. In Merlin's Furlong, a necromantic don runs a coven of witches. In The Mystery of a Butcher's Shop, the victim is minced into sausages and hung from hooks.

Ultimately, Christie remained safer and more controlled, while the complexity of Mitchell's uber-eccentric mysteries got the better of her. She tested the constraints of the murder genre by pushing them to breaking point, and by surprising too much she often disappointed – therein lies the clue to her canonical absence. But a flawed gem can still sparkle.