It is not easy to be a bookstore in the age of Amazon, especially when you're dealing with the low margins afforded by used books, as is Columbia's City's Bookworm Exchange. Stop by, and owner Jim Holmes will tell you that he is struggling mightily, like everyone else. Yet the doors remain open, and Bookworm is just the kind of rambling, comfortable space where you feel like losing yourself in words for an hour or two. The center of the store, as befitting a family neighborhood, is the children's section. There's a couch where kids can devour an installment of the Harry Potter or Percy Jackson series?or any other book they happen to find in the sizable collection. In the front, adults will find just as much to interest them, whether it be a New York Times bestseller, an early Martin Amis novel, or an obscure foreign writer you've dimly heard about?all at recession-friendly prices. If you feel like putting your book aside and just hanging out, there's a chess set at the ready. ? Nina Shapiro 4860 Rainier Ave. S., 722-6633