"Sisters," essays by Carol Saline, photographs by Sharon J. Wohlmuth. 131 pages. Philadelphia: Running Press. $27.50 This almost-coffee-table volume has become the "sleeper of the year." Published last October with brave but modest commercial ambitions, it has gone through multiple printings and back-order crises. Why? Each of the individual sets of 36 sisters offers a chance of personal identification, of glamour or an appeal to curiosity. As a whole, however, the book plunges deep and almost mysteriously into the nature and experience of what probably is humankind's most intense, complex, various and enduring personal connection. To read it is to weep, and to smile - a lot.