"I used to just let things happen to me," says the young widow who is the protagonist of Susan Shreve's appealing novel ["A Fortunate Madness".] "It wasn't necessary, I've found out." By the time Susanna MacPherson comes round to this enigmatic point of view, quite a bit has happened to her…. Mrs. Shreve describes these happenings with a sharp and sometimes twinkling eye nicely catching nuances of personality and habitat. In this impressionistic history, however, Susanna seems vaguer than her environment. We know what she feels, but are left sometimes wondering why. Nonetheless there are enough well-observed episodes in the novel to keep the reader engaged.