Book excerpt: With its rich literary tradition, the Midwest provides a wealth of opportunities for bibliophiles to retrace the steps of their favorite writers and characters. The Booklover's Guide to the Midwest is a treasure map in book form, pointing the way to the heartland's most interesting literary sites. Walk down the actual Main Street that Sinclair Lewis described in his classic novel, or among the gravestones that inspired Edgar Lee Masters' Spoon River Anthology. See Laura Ingalls Wilder's ''little House in the Big Woods'' and get lost in the very same cave that Tom Sawyer and Huck Finn explored. Visit Petoskey, Michigan, the setting of Hemingway's Nick Adams stories. Other poets and writers put readers in touch with pond life, sand dune architecture, Native Americans, and the great expanse of the prairie. Descriptions of each states' sites are arranged so that travelers can drive or walk from place to place with ease.