The last new New England book I recommended dealt with 50 icons. This time I'd like to call attention to a book devoted to only one. In Search of Motif No. 1: The History of a Fish Shack (L.M. Vincent, The History Press, Charleston, S.C., $21.99) is one man's account of the history and meaning of the frequently painted (and photographed) red shack on Bradley Wharf in the middle of Rockport harbor. Like all good tales it begins with a question and tracks through some pretty interesting turf to get to an answer. When Vincent first saw Motif No. 1 on the advice of a cousin, he found it attractive enough but couldn't see how it had become the subject of paintings by so many artists. As he unravels that tale, he takes the reader through a delicious amount of local art history populated by some pretty colorful characters. The author himself joins the cast as he tries to decipher the ''secret'' formula for the shack's shade of red. More than 200 footnotes notwithstanding, Vincent takes a light tone throughout--and even a mocking one on the subject of his own obsession. It all makes for a quirky, personable read.