There's a man giving away military secrets from a quiet warren at National Archives

John Taylor has spent 40 years revealing United States military secrets. He does this with enthusiasm. His best tidbits are written on small squares of paper, as if they were grocery lists.

''Here's a good one,'' he says, holding up a note. ''In World War II, in Europe, US secret agents were often named after plants. Basil. Nutmeg. Goldenrod.''

Mr. Taylor is not an antiwar activist. He presides over the National Archives Modern Military branch, a warren of rooms where casual chat can center on which SS regiment was commanded by Hitler's girlfriend's sister's husband.

He has worked at the Archives since 1946, becoming the man behind numerous famous books on World War II (such as Barbara Tuchman's ''Stillwell and the American Experience in China''), and even serving as the model for a character in a best-selling spy thriller.

''There are a thousand and one untold stories here,'' says Taylor, with once-top-secret documents strewn about his desk like so many old newspapers.

In the last several months, Taylor and his fellow archivists have been gleefully examining rich material. This summer, the CIA, after much prodding, shipped to the National Archives 450 boxes of memos, reports, and war diaries dealing with the Office of Strategic Services (OSS), the spy arm of US forces in World War II.

These old boxes, says Taylor, hold thousands of previously unknown details. The OSS used agents called ''chief whisperers'' to spread rumors in foreign lands. Saul Steinberg, famous for drawing New Yorker magazine cartoons, was an agency propaganda artist.

The OSS received intelligence reports on Japan from contacts in the Vatican. It helped the British run a ''black'' radio station that broadcast propaganda into Germany. This lavish station had a fountain in the control room and illuminated plastic furniture. The station transmitter was so powerful (with an effective power of 900,000 watts) that when it was turned on, light bulbs in nearby homes blew up.

''You never know what's going to pop up in these boxes,'' says Taylor, chortling as he moves off in search of more papers.

The newly released documents name the real names of many OSS agents and contain personal accounts of the war behind the lines - secrets that archivists say they have rarely seen before.

Take, for example, the story of Lt. Comdr. J. B. Roberts. Near the end of the war, Commander Roberts was para-chuted behind Japanese lines, in China, to accomplish sabotage with Chinese Nationalist guerrillas. The battery of Roberts's radio was dead on arrival. He received only half the 500 pounds of TNT he had been promised. The explosives were wet when delivered, and of an inferior type used for blasting tree stumps.

Still, Roberts and his Chinese aide, Chiu Wing, whose nickname was ''the Madman,'' managed to blow up numerous enemy trains.

''The Madman is turning into a legend,'' Roberts wrote in one of the weekly reports he somehow found time to file. ''Men beg to go with him.''

In the months preceding D-Day, the OSS divided German-occupied France into spy ''circuits,'' much as a manufacturing company splits countries into salesmen's territories. The circuits were named by someone with a feel for lilting words: ''Sacristan,'' ''Satirist,'' ''Wheelwright,'' ''Gondolier.'' Agents were dropped into each circuit, their job to work with the French Resistance in that particular area.

The newly released OSS material includes a green folio, titled ''W. Europe, Vol. 3, Bk. 1, Secret War Diary,'' in which these agents describe what life was like in occupied France. Claude Arnault, a civilian who worked for the OSS, was dropped into the ''Wheelwright'' circuit in late summer, 1944, and promptly captured.

''... they had found a German flag, German medals, grenades, a midget receiver in my suitcase,'' Arnault recounted, ''and they decided to shoot me at eight PM. However, I was lucky because the officer commanding the castle was called away, and he told the guards to keep me until he returned. I decided it would be safer to escape.''

Several days later, the castle where Arnault was imprisoned was bombed by the Allies. ''During the confusion,'' he said in his report, ''I managed to escape and ran to the river, which was about two miles from there, and crossed it. Then I walked about 50 miles through the woods so as not to be seen by the German soldiers searching the country.''

Another report tells of an agent who worked out of a Polish farmhouse. Some weeks after he arrived, the barn was commandeered by German troops. Unsus-pecting, the soldiers slept on a pile of hay that covered a secret radio. ''The whole period was a serious strain on the agent's nerve,'' says the report.

It is for historical detail such as this that writers and scholars flock to the Modern Military reading room, a small space with the atmosphere of an underground lunch counter. On an average day, says John Taylor, there are between 10 and 15 people riffling through military records at the archives, researching everything from PhD theses to screenplays.

German POW camps, the proposed US invasion of Japan, and the Nuremberg war crimes trials are popular subjects, Taylor says. So is anything dealing with the OSS.

William Casey, before President Reagan made him chief of the CIA, used to drop by and browse. Once a man who was a German spy in Latin America during World War II came in to read his file.

''I not only found his report, I found his photo,'' says Taylor.

John Taylor came to the National Archives as a freshly minted graduate of the University of Arkansas. He found himself wheeling around Army documents dating back to the 1800s and was quickly hooked. ''I liked it, from Day 1. I was fascinated by those records,'' he says.

He has been dealing with military documents ever since. Along the way he has acquired a top-secret security clearance (archivists in his department must have one) and worked with many well-known authors and historians.

He helped David Kahn with his ground-breaking tome on US cryptography, ''The Codebreakers.'' James Bamford, author of ''The Puzzle Palace,'' a 1982 book revealing activities of the shadowy National Security Agency, credits Taylor for finding ''the most obscure government document.''

Taylor urged historian William Langer to publish ''The Mind of Adolf Hitler, '' originally written as a government report. Barbara Tuchman, in the introduction to ''Stillwell and the American Experience in China,'' gives Taylor ''primary thanks'' for assistance in research.

The archivist even had a brief moment of glory as a thinly disguised character in a 1979 novel, whose author he assisted. The book, ''Provenance,'' by Frank McDonald, is about a modern-day search for art stolen by the Nazis. This is the Taylor character's big line:

''In this building we've got one-point-three million cubic feet of records. Do you know how many pages of material there are in a cubic foot?''

Taylor converses often with archivist colleagues overseas and works with many foreign researchers: On one recent day a Japanese film crew was for some reason leafing through Joint Chiefs of Staff records. Taylor has numerous friends who are OSS veterans.

With such contacts, and with his own knowledge of US documents, why doesn't he himself write some of the Archives' untold stories?

''I've been urged to,'' he says, ''but by 5 p.m. my eyes are too tired.''