The party at the home of the vice president of an unidentified South American country is well under way. Political dignitaries, the president of a Japanese electronics corporation, and a comely opera singer are unaware of the terrorists lurking in the air-conditioning vents overhead. The reader experiences a flutter of panic. But rest assured that Patchett (”The Magician’s Assistant”) is too subtle an author to write a hammy thriller. Ostensibly the story of a months-long hostage crisis, Bel Canto is more a meditation on the unlikely allegiances that form in extreme circumstances. An elegant businessman plays chess with his teenage captor; a soprano unites a divided room with Puccini. One approaches the final pages with a heavy heart for several reasons, not the least of which being that this fine read has come to an end.