Internet marketing is now a part and parcel of marketing. If you want to succeed, you have to go to where your customers will see you. Where else are you going to find everyone these days but on the Internet.
If you have a bookstore, you should advertise it online. And the good news is there are ways to keep marketing rather low budget, thanks to social networks. There's Multiply and Facebook, for instance. Twitter, of course, can be a effective way to do it too.
Blogs are also a great way to showcase your bookstore. You could feature new books and good sellers. You could also give out reviews on your blogs and websites. That could be a good start already.