According to the woman who has sold newspapers and periodicals to Capitol Hill employes for 28 years, Penthouse is the most popular over-the-counter magazine sold to congressional staffers. And the Senate intelligence committee and the offices of Edward Kennedy and Daniel Moynihan charge the most newspapers and magazines each month.

So says Ann Cooper, an Ohio native who came to Washington to take a hotel management course in the late '40s; she wound up spending most of her working life running a newsstand out of a small, white trailer perched hard by the old Carroll Arms Hotel.

"Oh, I could write a book better than old Bobby Baker," she says from a seat in her apartment where she's recovering from a broken hip. Will she write that book?

"Naww. Why should I?"

Cooper appears to enjoy a rent-free business; she moved into the trailer when the hotel in whose lobby she worked was renovated. And a staffer on the Senate Rules and Administration Committee says she's been "left there sort of by sufferance" ever since. While she laments the growth of the Senate's size, and gripes about women staffers who act too self-important, Cooper seems a favorite fixture: even when she was confined to bed, her business prospered, aided by friendly Capitol Hill police officers who manned the cash register.