Howard Penn Hudson, 91, whose Newsletter on Newsletters and seminars on newsletters -- as well as his book on launching newsletters, now in its third edition -- helped bring an oft-neglected corner of trade journalism into greater prominence, died Jan. 1 of a stroke at St. Francis Hospital in Poughkeepsie, N.Y.

Mr. Hudson did not invent the modern newsletter -- that honor goes to Willard M. Kiplinger, who started his first newsletter in the 1920s. But because of his timing and organizational ability, Mr. Hudson became the embodiment of the booming field.

From 1969 to 1999, he owned and operated a publication that, like a governmental committee on committees, is considered by insiders to be absolutely essential. His publication reported on newsletters for military history, houseplant growers, energy policy wonks, survivalists, business lawyers, tabloid fans and some even more arcane. A newsletter, Mr. Hudson said, is a tightly targeted publication supported by subscriptions rather than ads, no more than 16 pages and without a cover.

"Newsletters are so pervasive you don't even realize they're around," he told the Cleveland Plain Dealer in 1995. "You read them for information, then throw them away."

Mr. Hudson, who started his career as a traditional newspaper reporter, became a publisher for publishers in the 1960s.

"He liked finding the needs in an industry," said Paul Swift, who now owns and operates the newsletters' newsletter. "He did the same thing in the public relations industry when he started the Public Relations Quarterly (in 1955). He realized his Rolodex was only his Rolodex, so he literally walked from door to door in the National Press Building and started taking down names, addresses and reporters' [areas of coverage]."

That turned into "Hudson's Washington Directory," the comprehensive guide to the Washington press corps, which still is being updated and published after 38 years.

Mr. Hudson acquired the Newsletter on Newsletters in 1969, just as businesses and professionals realized they needed detailed, reliable information on the new Great Society laws and programs.

Mr. Hudson, who was born in Chicago, graduated from the University of Chicago and became executive director of the University of Chicago's Institute of Military Studies. He worked as a reporter for the United Press wire service and the Chicago Daily News in the late 1930s.

During World War II, he served in the 3rd Army in Europe, assigned to the Army Historical Division. His job was to write military histories, including a report on the Battle of the Bulge that earned him a promotion.

Mr. Hudson was then responsible for obtaining a detailed military history of combat operations from the 500 surviving German generals, which he did from the confines of a French chateau, Swift said. After the war ended, he was discharged as a lieutenant colonel and moved to Washington.

He worked for the National Planning Association and Ruder and Finn public relations agency in Washington, but his real career was as an entrepreneur. In 1965, he moved to Rhinebeck, N.Y., where he lived until his death.

He started a national association for newsletter publishers, now known as the Newsletter & Electronic Publishers Association. He also started an annual international newsletter conference, lectured and gave seminars three times at the Smithsonian Institution on newsletter publishing. In 1982, he wrote a book, "Publishing Newsletters."

His first wife, Mary Elizabeth Hudson, died in 1986.

Survivors include his wife of 15 years, Elaine Newman Hudson of Rhinebeck; four stepchildren, Barrett Newman of Washington, Seth Newman of Boston, Jennifer Newman of New Haven, Conn., and Dr. Lisa Newman of Binghamton, N.Y.; and four grandchildren.