It's always pleasing (except perhaps to veteran authors) when an unknown is plucked from the slush pile for a large book-and-movie payday. That's what has just happened to Linda Nichols, a Tacoma, Wash., housewife whose first novel, While You Were Out, went to Delacorte's Jackie Cantor and Leslie Schnur for a substantial six-figure North American rights deal brokered by Theresa Park of the Sanford Greenburger agency. Park, who has plucked previous winners from the slush pile (Nicholas Sparks, for one) said Nichols had written her a letter describing her book "so charmingly that it was irresistible." A surprisingly polished manuscript followed, and after Park sent it out on a recent Friday she got a call at home from an eager Cantor the following morning. By Monday the deal was made. Next, the movies: working through Howie Sanders and Richard Green of UTA, Park made a million-dollar sale to Universal/Dreamworks, with director Tom Shadyac (Liar, Liar) attached. The book tells of a disconsolate single mom who g s to the office of a pop psychiatrist promising a 21-day emotional overhaul. She starts pouring out her troubles to the nice, sympathetic guy she meets there. Turns out he's a contractor doing work in the office while the shrink's away. No prizes for guessing what happens next.