Since Acorn Books moved to the corner of California and Polk a few years back -- up from its Lower Polk Gulch digs -- it's taken a rightful place as one of the stateliest booksellers around Nob Hill, if not in the Bay Area. Classy enough to know its way around signed first editions but proletarian enough to sport shelves and shelves of battered paperback science-fiction and mystery novels, it regularly sucks in the foot traffic that oft-imposing antiquarian sellers don't. But for those who don't have much time (or money) to spend on literary avocations, Acorn's display window is a worthwhile trip in itself. There you'll find rows of photo books from a post-Triumph of the WillLeni Riefenstahl, vintage issues of Photoplay and Sports Illustrated, first-edition Herb Caen books, belle époque art tomes, and a slew of books on relatively obscure San Francisciana like the mechanics of the cable car. Lest you consider an impulse buy, though, it's worth noting that the front window is where Acorn puts its most expensive foot forward. One day, entranced by the signed first edition of Charles Schulz's A Charlie Brown Christmas in the window, we went inside to inquire about it. Nine hundred bucks, we were told. Uh, we'll be stepping outside.