All of Margaret Truman's novels are mysteries set in Washington DC (my hometown!) - and it's eerie how accurately she portrays places like The Kennedy Center, Union Station, Foggy Bottom, and my favorite, The Smithsonian:
http://www.amazon.com/Margaret-Truman/e/B000AP5KKK/ref=ntt_athr_dp_pel_pop_1

Admittedly, they may be on the thriller side of the murder mystery genre, but hopefully this helps!

Cupid & Diana is a charmer. Can't recall the author at the moment.

The Beautiful Things that Heaven Bears by Dinaw Mengestu is fantastic.

Fern Michaels, Game Over, 2010, 9780758235275; Shoe Addicts Anonymous, Elizabeth Harbison, 2010, 9780312348236; George Pelecanos,The Turnaround,2008, 9780316156479

If Georgetown will do, Prospect Street by Emilie Richards.

Back to the Bedroom and Naughty Neighbor - Janet Evanovich

Fool's Errand -- Louis Bayard
Man About Town -- Mark Merlis
The Zookeeper--Alex McLennan

Ward Just, "The Congressman Who Loved Flaubert," Jeff Greenfield, "The People's Choice," Allen Drury, "Advise and Consent" (fans of Mad Men, take note!). For readable slush with an insider touch, Sally Quinn's roman a clef, "Regrets Only." For keen-eyed memoir and nonfiction, Marjorie Williams, "The Woman at the Washington Zoo." For period memoir, Barbara Howar, "Laughing All The Way." For social history, try David Brinkley, "Washington At War."