WASHINGTONIAN BARBARA HOWAR is in the process of being sued by Random House for failing to return a $50,000 advance for her second novel, Setting Things Straight, which they say she did not complete to their satisfaction (a contractual phrase publishers are more inclined to employ, nastily, in these straitened times). Now, Linden Press, where Setting Things Straight next wound up (it was announced in their '81 fall-winter catalogue yet never came out), is saying that "the book's on hold indefinitely." Linden publisher Joni Evans, who recently wrote in a magazine article that executives should return phone calls the same day, didn't . . . There've been group-authored novels before (who remembers "Penelope Ashe's" Naked Came the Stranger by a bunch of lascivious cut-ups from Newsday?). Now five Washington Post staffers plus a former Watergate prosecutor have tried their hands at one. "There's no formal agreement between us and Simon and Schuster," says Bob Woodward, one of the conspirators in fiction. Together on a 51/2-day sailing trip from here to Bermuda, he, reporters Patrick Tyler and Christian Williams, assistant managing editor Tom Wilkinson, art critic Paul Richard and lawyer Carl Feldbaum concocted a spy thriller, dictating it into a tape recorder. "We have a draft, about 300-400 pages, and it's being transcribed right now." The tentative title of this effort? Tilt . . . For summer beach-goers, Perigee Books has out a daunting volume called Castles in the Sand. Identified as "10 projects that can be built in one to six hours," it's a collection of architectural wonders in sand, including the Capitol and the White House, created by a "master castler" from the Jersey shore named Michael Dipersio. Of the executive mansion castle, the text by Jeffrey Shear and Steven Schneider says, "it does not have as much detail as the Capitol, and it is not nearly so high." . . . One would expect to find stars in a book by Jack Valenti, who for the past 16 years has been the president of the Motion Picture Association of America. But, actually, Speak Up With Confidence: How to Prepare, Learn and Deliver Effective Speeches (Morrow) contains only a few, and an odd assortment at that --Johnny Carson, Don Rickles, the late Jack Benny, Arlene Alda. However, Valenti, a former LBJ aide, does make one comparison that would probably not occur to someone in another line of work. Here's how he describes President Ferdinand Marcos of the Philippines: "a compact, muscular man with heavy black hair and a flat-planed face, vaguely resembling Jack Palance."