The review "Heat-Moon Plumbs the Plains," Oct. 16, of William Least Heat-Moon's new book "PrairyErth" recounts a tale about Congressman "Sockless" Simpson who, when asked why he misspelled the name of his own home town, replied, "I wouldn't give a tinker's durn for a man who can't spell a word more than one way."That same sentiment was expressed some years earlier by Mark Twain, whose words were: "I don't give a hoot for a man who can spell a word only one way." Beatrice Duffy, Glide, Ore. Letters are welcome. Only a selection can be published, subject to condensation, and none acknowledged. Please address them to "Readers Write," One Norway St., Boston, MA 02115.