Swartzwelder’s specialty on “The Simpsons” was conjuring dark characters from a strange, old America: banjo-playing hobos, cigarette-smoking ventriloquist dummies, nineteenth-century baseball players, rat-tailed carnival children, and pantsless, singing old-timers. After leaving the show, in 2003, Swartzwelder wrote and self-published the first of his thirteen novels, all but two of which feature one of the most wonderful creations in printed comedy: Frank Burly, incompetent private eye and occasional time traveller. None of the books run more than a hundred and sixty pages; all are packed, like a dense star, with more material than seems physically possible.