Epilogue Books has been a great addition to Market St in the last couple of years. This spacious store offers an active calendar (readings, discussions, open mics, visiting authors etc.), an in-house coffee shop, and a great kids section.
The folks here do not mess around... they buy and sell books to people who really love books. The prices are great (I often drop in to pick up a book with $5 in my pocket) but their books are in incredibly good condition on average. Epilogue doesn't pay money for used books, but they do trade *generous* credit for books in good shape. (Note: I have some books I've bought at Library book sales in the past and found that the folks at Epilogue won't buy them.)
If you are looking to do gift-buying for a bibliophile, or looking to treat yourself. I highly recommend Epilogue, because they have books in all price-ranges and an incredibly wise, friendly and helpful staff.