As far as I know, there are no bookstores anywhere right now. Market is one way to buy books (kiosk has only magazines), but the chance to find a book seller there is really low. The only reliable way I've found to get books is to hang out with Artem (one of the school nerds) and borrow random fiction from his apartment.
You're better off going to a library, although I haven't found a practical use for books. I guess they can raise Sveta's intelligence, but paying attention during lessons does that too.