The National Book Store (you've never heard of it, either?) has been feeling somewhat neglected since it opened just in time for the Bicentenial at the National Visitors Center at Union Station, and that's a shame.

It is a small, but valuable resource available to Washington residents that somehow has escaped much notice. You migh hurry right by on your way to catch a New York Metroliner.

The store believes that it has the best collection available for sale anywhere of books and pamphlets on America's national parks, making it the ideal place to visit when you are planning a trip to one of them.

The parks collection begins with a motorists' guide to Acadia National Park in Maine ($1.50) and concludes -- yards of shelves later -- with "Discover Zion," an area of spectacular desert and canyon scenery in southwest Utah ($5.95).

In between, you might want to sample:

Folk Medicine of the Mammouth Cave Area ($1.50).

The Story of Hoover Dam ( $1).

Hikers Guide to Glacier National Park ($4.95)

The Wolves of Mount McKinley ( $4).

The Nature of Shenandoah ($2.35).

To give another example of the breadth of the collection, it contains at least six books on different aspects of Isle Royal National Park, the northwoods island park in Lake Superior. Included are studies on the geology, forests and trees, birds, sport fishing, wild flowers and "The life."

Many are books of color photographs, the kind you might buy at the park itself as a souvenir of a summer vacation.

"We don't have everything," says Franziska Hecht, executive secretary of the Parks and History Association, which manages the National Book Store and 17 other smaller book facilities at parks and monuments in the Washington region. "But none of the other national parks has tried to collect as much as we have."

The bookstore's function, she says, "is to tell the story of America -- its history, the people and the country." The local association and more than 50 similar associations throughout the country -- all in cooperation with the National Park Service -- have published books on the country's parks, many of which may not be available except at the park or the bookstore here.

Among the non-profit association's other book outlets are those at Antietam National Battlefield, Glen Echo Park, Ford's Theater and Rock Creek Nature Center. The busiest is at the Lincoln Memorial, where "In This Temple," the story of the building of the memorial, is the big seller.

Books and literature at these other stores center mostly on the history and other aspects of that particular site. The bookstore at the National Aquarium, for example, carries a large number of books about fish.

Proceeds from the book sales go either for maintenance of the bookstores or as donations to the National Park Service. Funds have gone to the Arlington House to research and obtain fabric to reupholster chairs and to the Mount Vernon bicycle trail for mile-markers.

The Nation Book Store also contains a large collection of books of American history, biography, social and current issues, and books on the Washington region. Big sellers include guidebooks and maps to the Nation's Capital.

Decorating the bookstore walls are huge national park posters selling for under $5. These have been quite popular, says Hecht.

The bookstore operates a mail-order business for its national park publications. A catalogue can be obtained by sending $1.25 to the National Book Store, National Visitors Center, 50 Massachusetts Ave. NE., Washington, D.C. 20002.