Charles Rosen, a pianist and polymath who became known as much for his insightful writing and talking about music as for making it, died of prostate cancer Dec. 9 at a New York City hospital. He was 85.

His publicist, Constance Shuman, confirmed the death.

Mr. Rosen’s encyclopedic knowledge made itself felt on and off the stage for more than five decades. His concert-hall repertory extended from Mozart to Debussy to Elliott Carter, making it so hard to pin him down that his recording label, CBS, eventually gave up on his contract. His versatility served him admirably as a music scholar.

Having taken up writing in a kind of self-defense — “to begin with I wrote just to keep nonsense off my record sleeves,” he once said — he won a National Book Award for his first major book, “The Classical Style” (1972).

The book was a thoughtful, erudite and soundly argued study of the music of the three principal composers of the Classical period: Haydn, Mozart and Beethoven. Hailed as an instant classic, “The Classical Style” remains the definitive book on the music of that era — which spanned the late 18th and early 19th centuries — which did so much to define the Western canon. Filled with examples of notated music, the book was at times a demanding read for a lay audience.

Charles Rosen, a pianist of towering intellectual gifts, died Dec. 9 in New York. He was 85. (Peter Schaaf/COLUMBIA ARTISTS MANAGEMENT)

“The Classical Style” launched a writing career that extended through several other volumes. Mr. Rosen also became a prodigious contributor to the New York Review of Books, writing about classical composers but also subjects including philosopher Isaiah Berlin, the cookbook writer Elizabeth David, Parisian art exhibitions and the Restoration-era English playwright and poet William Congreve.

In a 1977 New York Times interview, Mr. Rosen called writing his hobby.

“At the piano, if you practice 10 hours a day, then you have no time to write,” he said. “I really can’t practice more than about four to five. I can play the piano for eight to 10 hours a day, but I can’t practice for that long. So I have to do something with my time.”

Charles Welles Rosen was born in New York City on May 5, 1927. As a child, he attended the prestigious Juilliard music school in Manhattan and then studied with the pianist Moriz Rosenthal, who had been a pupil of the 19th-century Hungarian pianist and composer Franz Liszt.

The connection to Liszt put Mr. Rosen in a direct line with the grand romantic tradition that was later to become the subject of one of his books.

Rosenthal was no slouch at the kind of incisive one-line put-down that Mr. Rosen was to incorporate into his writings. He also connected his young student with the musical and intellectual life of New York.

When Mr. Rosen matriculated at Princeton University, he said in interviews, he already knew the whole music department and what it might have to teach him. Instead, he majored in French literature, eventually earning a doctorate from Princeton in 1951. He then won a Fulbright scholarship, going to Paris to study 15th-century musical manuscripts.

Meanwhile, his piano career continued with high-profile performances at Town Hall in New York and the release of his first recordings.

Of Mr. Rosen’s rendition of Johannes Brahms’s Variations on a Theme of Paganini, Time magazine wrote in 1952 that the 25-year-old pianist “swept along like a fresh breeze in a musty corridor, slamming doors on heavy-handed traditions and uncovering the fine old structure. Listeners heard more details than they believed possible, played in tones of pastel shading.”

A year earlier, Mr. Rosen made a small sensation with the first complete recording of Debussy’s Etudes, and his record label thought of him as a French specialist.

The connection to France sustained him throughout his life, and he maintained an apartment in Paris, as well as a residence in New York. But he defied categorization, recording Bach, Beethoven and Pierre Boulez, who along with Carter, a friend who died last month at 103, remained one of his artistic touchstones.

Because of his allegiance to Boulez and Carter, Mr. Rosen had a reputation for playing contemporary music, but his taste was for the kind of smart modernism that these composers represent.

“There is still a question about how long the modernism of the 1950s is going to last,” Mr. Rosen told the London Guardian in 2011. “The problem is that in order to absorb any difficult style you have to hear the pieces several times. That was true of Mozart, most of whose work was considered difficult at the time. . . . You really have to hear them well played several times, and there are very few music lovers who’ve heard a piece by Boulez more than once. That’s the fact of it. So will it last? I don’t know. Some music doesn’t.”

At times, critics found that Mr. Rosen’s intellect weighed too heavily on his playing. A Time reviewer found one of his early recordings to be “clean, fiery and absolutely unsentimental.”

There’s no doubt, however, that his analytic mind formed his distinctive approach to music. And there is little argument that his reputation as a thinker, certainly in the final decades of his life, tended to eclipse his reputation as a pianist. This development was perhaps inevitable, since he was one of the pre-eminent music thinkers of his generation, while there were plenty of other pianists to share the pianistic crown.

In 1980, Mr. Rosen gave the Charles Eliot Norton lectures at Harvard, later published as the book “The Romantic Generation” (1995), which analyzed the developments in European music from the death of Beethoven in 1827 to the death of Chopin in 1849; the book came with a CD recording by Rosen.

His other books included works on composers Arnold Schoenberg and Carter. In addition to his writing and music, he taught at the State University of New York at Stony Brook and the University of Chicago, where he was a professor emeritus at the time of his death. He left no immediate survivors.

In February, President Obama presented him with a National Humanities Medal, whose citation noted his “rare ability to join artistry to the history of culture and ideas. His writings — about Classical composers and the Romantic tradition — highlight how music evolves and remains a vibrant, living art.”

“The task of the critic,” Mr. Rosen told the Guardian in 2011, “is to tell people the way to listen to music so that they get more pleasure out of it through enhanced understanding. Basically, there is no difference between understanding and pleasure. If you listen to a piece of music and it makes sense to you, then you generally like it. I’ve been so lucky to have had a life devoted to playing music and trying to make sense of it. It has proved remarkably satisfying.”