IN AN AREA laden with well-known historic landmarks, the Collingwood Library and Museum on Americanism probably goes unnoticed to many who glimpse its gates from the Mount Vernon Parkway. But the stately white building houses a treasure trove of information for American history buffs.

A sign at the entrance to the nine-acre site welcomes visitors to "this scenic and historic portion of George Washington's River Farm dedicated to patriots and scholars of our nation's past, present and future." Collingwood's live-in curator, Peter W. Pedrotti, agrees with a Washington biographer who speculates that Washington bought 1,806 acres from William Clifton in 1760 to have land on which his new wife's 50 farm slaves could work. The property also included a fresh-water bubbling spring, from which Washington profited with sales of drinking water to sea captains, Pedrotti says. Washington had the original Collingwood building constructed as a home for Sam Johnson and his family, who sold the spring water and operated a ferry that crossed the river to Maryland.

Beginning in the 1850s, the building served as a farmhouse for Quaker brothers who bought 900 acres from Washington's grand-nephew, Charles Augustine Washington, Pedrotti says. The family added rooms and the home grew into its current Southern Colonial style, complete with a colonnaded front porch.

The Foundation for Collingwood Library and Museum on Americanism took possession of the property in 1977. The building, heavily vandalized after standing vacant for 2 1/2 years, required a $400,000 restoration.

The library's collection, not for lending, deals mainly with U.S. history and culture, from American Indian lore to the Civil War to presidential biographies. Most books are donated by people who belong to the 9,000-member National Sojourners Inc., a Masonic veterans group that serves as the Collingwood Foundation's parent organization.

The Revolutionary era and the Civil War are probably the library's most-representated topics, the curator says. Among the collection's volumes on George Washington is a set of 36 of the 39 books of the general's writings authorized by the Bicentennial Commission for the celebration of his 200th birthday.

Among the collection's oldest works are a children's reader published in 1836 and two books bearing the same title: "Washington and His Generals," written in 1875 and 1885.

"I know I've got books here that no school library would possibly have," says Pedrotti, who visits local schools to try to drum up students' interest in doing research at Collingwood.

Jackets from books written about each of the 50 states hang atop the shelves in the library's largest room, lit by chandeliers and furnished with an overstuffed sofa and chairs in front of a fireplace guarded by a portrait of George Washington. Researchers can spread out materials at a large table in the center of the room.

A small, separate section at the rear of the main library houses a card catalogue and a wall of shelves holding genealogical periodicals and 650 volumes belonging to the Society of Mayflower Descendants of the District of Columbia. The D.C. society's library is open to the public during Collingwood's hours, and its librarian, genealogist Jane Barrett, visits from 10 to 2 Wednesdays to assist area residents interested in tracing their lineage to the Pilgrims.

"We tend to use as recent and as modern information as we can, because the Mayflower Society itself is very, very strict about the lineage," Barrett says. "Every generation has to be proved, and it has to be proved by firm information, not just stories."

Barrett, a descendant of Pilgrim Edward Fuller, says that people wishing to learn whether they have Mayflower ancestry should first have an idea of the passenger's name. Of the 101 people who came over on the Mayflower, 50 died the first winter, Barrett notes. "Of the 51 who survived the first winter, there were only 23 . . . who left descendants."

Although Barrett can trace a person's lineage if so requested, she usually explains the steps and offers help if someone gets stuck. Some complicated family trees take as long as six months to trace, she says.

Many Collingwood visitors wander in out of curiosity after seeing signs for the Museum on Americanism. Pedrotti directs them upstairs, where glass cases in the small museum hold a variety of objects, including circa 1850 portraits of George and Martha Washington done in Eglomise, a method of painting on glass. A Sioux Indian chief ceremonial headdress stands out in the collection of Native American artifacts, donated by a man whose father was an administrator for the Bureau of Indian Affairs. The uncatalogued items include pottery as well as woven and beaded pieces.

The museum display also features state flags, a replica of Paul Revere's Old North Church lantern, National Sojourners' memorabilia and a copy of the Magna Carta cast in 24-carat gold.

Collingwood hosts 40 to 50 marriage ceremonies and wedding receptions annually, in addition to meetings of fraternal organizations, holiday gatherings of local businesses and occasional sessions of congressional committees. Visitors can wander the landscaped grounds, which include a path that heads toward the Potomac.

Pedrotti is often called upon to answer questions from visitors or researchers who call or write requesting information. Recently, people have inquired about a portrait artist who worked locally and a minister who signed the Declaration of Independence. One man called long distance to ask Pedrotti what size shoes Thomas Jefferson wore. The curator researched the question and found that the un-sized shoes during Jefferson's day were custom-made by cobblers in a square-toed style enabling people to extend wear by alternating the shoes from one foot to the other.

COLLINGWOOD LIBRARY AND MUSEUM ON AMERICANISM -- On East Boulevard Drive off the Mount Vernon Parkway, halfway between Alexandria and Mount Vernon. 703/765-1652. Open 10 to 4 Mondays and Wednesday through Saturday, 1 to 4 Sundays and other times by appointment. Free admission.

Mary Jane Solomon last wrote for Weekend about area pumpkin patches.