Pierre Boulez’s Livre pour quatuor is among the most complex chamber music works of the 20th century. At the same time, the score—left unfinished by the composer—also has a fragmentary quality. The JACK Quartet, regularly heard at the Pierre Boulez Saal over the last few seasons, performs individual movements from Boulez’s only string quartet, pairing them with recent works by Eva­-Maria Houben and Anthony Cheung, among others, and the String Quartet in Four Parts by Boulez’s contemporary John Cage.