Leeson's deep regard for Mozart enlivens this close, readable look ("the world's longest program note") at the Requiem, Mozart's final composition. How an incomplete set of "musical blueprints" became "one of humankind's greatest treasures" after the composer's untimely death is the problem with which the author, a former professional bass clarinetist and a Mozart expert, concerns himself. There have been plenty of mysteries -- from the Requiem's commissioning, through its completion, to the machinations of Mozart's wife, Constanze, in the wake of his death -- many solved in part by other scholars: as Leeson acknowledges, there is no shortage of Requiem literature. But this is the Requiem as a story. Leeson explains the three principal problems with the Requiem as Mozart left it -- insufficient orchestration; a Lacrimosa barely begun; a composition too short for liturgical purposes -- and shows how the composition was transformed after his death. He gives a thoughtful discussion of Franz Süssmayr's ability to rise to the occasion of finishing Mozart's work (after Joseph Eybler withdrew) and the public's desire to believe that only Mozart could compose such a piece. Leeson's writing is concise, persuasive and logical. Scholars will wish for citation in the form of notes, indexes and bibliographies, but general readers -- at whom Leeson aims -- won't miss them and will enjoy instead a compelling story.