NOBODY THROWS a book party when a writer really needs it - on the day she signs a contract and has 15 chapters to go. Nonetheless, they gave Phyllis Theroux a we-know-you-can-do-it party when she signed with William Morrow to write her first book ("an affectionate story of my family," she calls it).

The party took place in New York one evening in early May and all the editors who bid on the book were there. "My agent, Ann Buchwald, sent the first chapter - a tattered document full of coffee stains - to different publishers in New York", she says. Six houses were interested so the book was put up for auctioned by telephone. When the gavel came down, Phyllis Theroux had three identical offers (for a figure her agent calls "hefty") and was forced to decide among them. "I felt like a sorority pledge during rush week." She chose William Morrow but invited the five also-rans to her contract-signing party.

Home again in Chevy Chase, where she shares a house with three children and four boarders and has been writing for newspapers and magazines for the past five years, she is "back by the ashes." It was "a wonderful party," she says, "but the ball is over. I have to get down to hard work."