Don't weep for Newt Gingrich. Sure, the new king of the Hill gave back that $4.5 million advance from his two-book deal with HarperCollins, but his agent expects the Speaker of the House to make a bundle in royalties when the publisher ships 750,000 copies of his tract, To Renew America, next fall. But will he? An informal canvass reveals that booksellers aren't guaranteeing a landslide. ''The publisher's expectations are going to be high and I think they're not going to be met,'' says Richard Howorth of Square Books in Oxford, Miss., a place where works by William Bennett and Rush Limbaugh have done well. Rick Simonson of Seattle's Elliott Bay Books Co. echoes Howorth's view: ''Most books by past or present politicians Tip O'Neill, Ronald Reagan haven't sold much.'' In other words, Gingrich beware: Fortunes in Washington, as in the book biz, can turn on a dime. ''I cannot give away a Clinton book,'' says Penny Davies of Earthling Bookshop in Santa Barbara, Calif. ''How long has he been in office?''