Mrs. Scherman is an enthusiast of untrodden ways, but both the islands celebrated here -- Grand Manan at the mouth of the Bay of Fundy and Sanibel off the gulf coast of Florida -- offer populations. Manan's 2700 people are mainly fishermen and cannery workers, Baptist and self-contained. And the island,""lonely, dangerous and beautiful . . . belongs to itself."" The author and her husband happily, if cautiously, roamed about observing birds, plants and bleak, tumultuous scenery. Sanibel on the other hand is threatened by development and the tourist trade; and whereas Grand Manan's history is characterized' by sparse and struggling populations and offhand political swaps between the Great Powers, Sanibel has been a ""haven for disreputable activities"" -- pirates, drug smugglers, etc. As for the tourist concentration, Mrs. Scherman dreams of a hurricane that will ""wipe it all off."" But there are still areas of natural beauty and there are discussions of flora, fauna and a lively chapter about shelling. Illustrated with photographs, this is pleasant for bird watchers, nature buffs and rugged vacationers.