FOR ABOVE-AVERAGE selections of Washington guidebooks and maps, there are three good commercial outlets, one of them notably well-tended, plus the Smithsonian bookstore at the Museum of American History. There's also a free information center run by the Washington Convention and Visitors Bureau.

The standout bookstore is Travel Books Unlimited (4931 Cordell Ave., Bethesda), a five-minute walk from Metro's Red Line and leagues ahead of the competition in its comprehensiveness. Travel Books stocks every book mentioned in the accompanying story -- plus half a dozen picture and history books on Washington, plus an additional 30 volumes on the mid-Atlantic region. And the aforementioned takes up, oh, roughly a tenth of the store's shelf space; in general, if it's travel related and published, owner Rochelle Jaffe has it -- and knows what's in it.

It's only since Jaffe opened the store two years ago that she's become a world traveler; what she's been most of her life is a serious armchair itinerant -- a reader. This explains the table and chairs, available in the back to anyone so engrossed. Just don't miss your plane, sitting here. Call 951- 8533.

The others:

THE MAP STORE -- 1636 I St. NW. The best selection of maps -- from basic tourist to top-of-the-line topographical -- in the city. A lesser, but decent, spread of guides to D.C. and environs. A good location, adjacent to the 17th Street exit of Metro's Farragut West stop. A knowledgeable, friendly staff. A phone number: 628-2608.

TRAVEL MERCHANDISE MART -- 1425 K St. NW. Convenient to the downtown/Mall tourist circuit (next to the U.S. Passport Office, coincidentally enough, and a block north of Metro's McPherson Square stop), this place is equal parts travel agency, luggage/travelware outlet and bookstore. That last third holds a very good selection of Washington guides and maps. 371-6656.

SMITHSONIAN BOOKSTORE -- At the Museum of American History, Constitution Avenue NW at 14th. The most convenient bookstore for Mall visitors, though the selection of guidebooks is heavy on Smithsonian- produced works. 357-1784.

TOURIST INFORMATION CENTER -- In the Commerce Building's Great Hall, Pennsylvania Avenue NW at 15th (open 9 to 5 Monday-Saturday, Sundays also from April to Labor Day). No guidebooks here -- nor signs indicating the entrance, which is directly opposite the grassy southern slopes of Pershing Park -- but plenty of help. The help, which says the signs will be up soon, includes staffers from the Washington Convention and Visitors Association, which runs the place, and from Travelers Aid and the International Visitor Information Service (IVIS). It also includes free brochures, events calendars and theater schedules of all types. The best free offerings are: "Washington DC: A Capital City!," the WCVA's helpful basic directory of phone numbers, addresses, hours and hints, including a small but forthright tourist-area map; "Calendar of Events," a theater-dominated entertainment brochure revised quarterly; and "Washington Weekends," a digest of D.C. hotels' cut-rate weekend packages. 789-7000.