Charles Rosen, the pianist, polymath and author whose National Book Award-winning volume “The Classical Style” illuminated the enduring language of Haydn, Mozart and Beethoven, died on Sunday in Manhattan. He was 85.

The death, at Mount Sinai Hospital, was of cancer, said Henri Zerner, a friend of many years.

Published in 1971, “The Classical Style” examines the nature of Classical music through the lens of its three most exemplary practitioners. Given that these titans were working with the same raw materials — the 12 notes of the Western musical scale — as the Baroque composers who had preceded them, just what was it, Mr. Rosen’s book asked, that gave their music its unmistakable character?

The answers, he concluded, could be gleaned from a penetrating analysis of the structure of Classical compositions. It was precisely this structure that his book, through a painstaking unraveling of Haydn’s string quartets, Mozart’s comic operas, Beethoven’s piano sonatas and other seminal works, sought to make plain.

Though some critics took the book to task for its heavy reliance on musical notation (a work aimed at a general readership, they argued, should be accessible even to those who could not read music), most praised it as a masterly work of synthesis.