MANLIUS, N.Y. — When Boy Scouts reverently say William Hillcourt wrote the book on Scouting, it's the literal truth. Scout's honor.

Next to Lord Baden-Powell, who founded the Boy Scouts in England in the 1890s, the 91-year-old Hillcourt is the most widely known figure in Scouting.

Hillcourt is the principal author of the official Boy Scout Hand Book, which has sold nearly 32.9 million copies since its first printing in 1910. The ninth edition alone, which came out in 1979, has sold 4.4 million copies.

The first six editions of the handbook were written by several authors, although Hillcourt was the major contributor to several editions. Beginning in 1960, with the seventh edition, he became the sole author.

Hillcourt also influenced generations of boys through the tales he told in Boys' Life--Boy Scouting's magazine--as Green Bar Bill, the know-all patrol leader.

"His enthusiasm for the outdoors and the Scout's life was always a part of his writing," says William B. McMorris, editor-in-chief of Boys' Life.

"He said, 'Here's what you can do. Get out and do it.' His enthusiasm reached to all Scouts."

Hillcourt will tell you the same thing, but there's little swagger behind his words.

"Mine is the story of a serendipitist. I set out to become a pharmacist and became a writer of handbooks for boys," says Hillcourt, whose voice still betrays his Danish heritage after nearly 70 years in the United States.

Robert Hood, who retired in 1986 after 22 years as editor-in-chief of Boys' Life, says Hillcourt "knows everything about Scouting. He knows the bread-and-butter about teaching the Scouting crafts. He's a link to Scouting's past."

Hillcourt was 10 when he read Baden-Powell's "Scouting For Boys," which had just been translated into Danish. He immediately became a Scout.

Ten years later, he traveled to London to attend his first International Jamboree, where he met Baden-Powell. The meeting started a lifelong friendship that resulted in Hillcourt writing the definitive biography of the British general, "Baden-Powell--The Two Lives of a Hero."

Hillcourt had become a pharmacist by 1924, graduating from the Copenhagen Pharmaceutical College. But his yearning to write continued. During his college days, he edited the monthly magazine for Danish Boy Scouts and by the time he was 23 he had published his first novel, recounting his experiences camping on a desert island on Denmark's largest lake.