Ironically, having to compete against a Starbucks is more likely to increase your initial business than decrease it. I'm not sure it would even put a cap on your total business, since the goal of the coffee is really to obtain book customers.
There's exceptions if you're talking about a well established coffee shop that's been around for a while, but most coffee shops will first attract people that happen to be in the neighborhood for some other reason. It takes a long time to become an attraction that actually draws customers to a neighborhood just to visit your coffee shop.
Starbucks already has a national reputation and customers will visit it if it's anywhere close to where they're at or to the route they follow to get somewhere else. The Starbucks draws customers to the neighborhood that happen to be interested in coffee. When Starbucks is too busy, or just because some of the potential Starbucks customers happen to notice they could get both coffee and books, the local coffee shop is going to pick up new customers at a faster rate than they normally would.
Of course, the coffee has to taste as good as Starbucks and the book selection has to be good, but Starbucks isn't quite as likely to put the local coffee shops out of business as people would think and I think that would be doubly so in this case when the primary business is selling books.
Competing against a store that sells new books might not be that disastorous, either. I'm less sure about a used book store competing against a B&N or Borders, but they might actually be complementary businesses. The main resource you'll be competing for is time - does a customer have time and inclination to browse both a new book store and a used book store on the same shopping trip.