Capitol Hill

``Keeping up appearances''--that's what Washingtonian Sheridan Berk learns her hometown is all about in Mann's ( Rumors , stet this sales info/pk which has 145,000 copies in print) fast-paced new thriller. Sheridan seems comfortably ensconced in the life and structure of the nation's capital. She has a PR firm, a townhouse on Capitol Hill and very personal contact with the White House: for three years she has been having an affair with chief of staff Baxter Caldwell. Then suddenly everything begins to fall apart, and Sheridan is forced to see that murder and betrayal lie behind the facade of power that has seduced her since childhood. Often Mann's writing becomes didactic and insultingly simplistic, but the vital characters grow and become stronger as the plot unfolds. Sheridan and her journalist housemate Noriko Sonada are '90s women with '90s dilemmas--career vs. family, independence vs. companionship, love vs. friendship. This novel is about Washington but has nothing to do with the public side of government; Mann shows us instead private foibles and scandals, which make for a fun and absorbing yarn. (Mar.)