If you have more frequent flyer miles than you can possibly use or you never seem to accumulate enough for a free ticket, Northwest Airlines now has a new option for customers. The airline will now let passengers use their extra miles to buy books, wine and lots of other retail products. Northwest is one of six carriers to join with the e-commerce company, milepoint.com, which offers links to more than 100 retailers that accept frequent flier miles as cash. Northwest spokeswoman Mary Beth Schubert says the web retail site is easy to use.