Seon Manley's Discovery of Long Island begins with the glacial age (L.I. is ""a result of deposition and erosion""), moves on to the days when the Mohawks prevailed (they ""sometimes forced their liberty on others""), to the Dutch settlers, through the Revolution, when the British held it and Nathan Hale became ""a brilliant failure"" there. There followed the pursuits of the nineteenth century on land and sea, farming and whaling, the social whirl of the 1920's recorded by the North Shore's F. Scott Fitzgerald, which has slowed to a more leisurely pace today. It tells of sea and land and people--Lyman Beecher who was ""bay in spirit and nor'easter in the pulpit,"" Quaker leader Hodgson whom Governor Stuyvesant gave ""something to think about besides 'gladness',"" artist William Sidney Mount, ""poet laureate"" Walt Whitman, and other great men, Charles Lindbergh, Teddy Roosevelt, Albert Einstein. There are the islands with their histories--Fire and Fisher and Gardiner's where ""Lord John"" dealt with Captain Kidd. There is the Big A and Belmont and another kind of Race Week (boats) and sport (duck hunting). Mrs. Manley communicates all this with agreeable ease and at an agreeable level of informativeness...say, just right for the ""one out of every four rail commuters in the entire countryside (who) rides the Long Island Rail Road.