Across the U.S. preservationist efforts are making it increasingly possible to step into the past without making a pilgrimage to Sturbridge Village or Williams-burg. Here, in a straightforward state-by-state roundup, are the towns that now boast historic districts, with a descriptive listing of the notable buildings in each. Cromie doesn't conjure up the setting, a small minus, but she's alert to what's unusual--the Penobscot (Me.) Expedition Site, one of the few commemorating an American defeat; Old Dry Road Farm, near Reading, Pa., a working slice of rural life--as well as informative on salient historical facts. Those living at a remove will also learn about such large-scale local efforts as Old Wisconsin World (a museum village slated to include some 20 ethnic settings) and Grant-Kohrs Ranch National Historic Site (a re-creation of early cattle-ranching days). There's less about highly-publicized California than you might expect--and refreshingly much about low-profile Ohio and Indiana, where lots of smallish spots await the inquisitive, unhurried traveler. The advice, such as it is, is judicious (Vermont Country Store ""is high-tourist trappery but worth a drop-in visit""; many tours serve multi-attraction Charleston). But the susceptible reader won't require more than the brief mentions.