Martin Mayer, an author, journalist and critic of remarkable diversity who wrote more than 40 books and hundreds of articles for laymen that demystified lawyers, banking, thorny school problems and the raptures of classical music, died on Thursday at his home on Shelter Island, N.Y. He was 91.

Mr. Mayer’s son Thomas said the cause was respiratory complications of Parkinson’s disease.

Martin Mayer called himself an old-time freelancer, batting out 1,000 words a night before dawn. But for a half-century, Mr. Mayer was a Renaissance man of letters, taking readers on behind-the-scene tours of Wall Street, Madison Avenue, the practice of law, the solemn delights of a Spanish guitar and the tangles of a racially divisive New York City teachers strike.

Along the way, he churned out three novels; wrote columns for Esquire magazine; was a music critic for a British journal; wrote for The New York Times, The Wall Street Journal and a dozen other periodicals; composed reports for the Ford, Carnegie and Kettering Foundations; was chairman of a local school board in New York; and served on White House panels in the Kennedy and Reagan administrations.

A free-spirited son of two New York City lawyers, Mr. Mayer entered Harvard in 1943 at age 15 under an experimental program. It was a mistake. He and his roommate exploded a chemical concoction outside a college gymnasium as Naval trainees ran up one morning. (No one was hurt.)