Think of your business as having 2 knds of products: traffic items and margin items. traffic items are those that get people in the door and margin is what keeps you in this year's fashions. coffee can do both.
Dunkin Donuts sells 13 donuts for every cup of coffee, but based on models we've run, they make about 60 percent of their profits on coffee.
Comics would be... traffic items - branding for your store to differentiate it from other coffee shops. They might also be anti-traffic items if people look in the store and have an image of 30 years olds living in their mom's basement IMing their krew about Deep Space 9. If your city has the population to support a comic shop (Boston does) you can probably add in a coffee bar. You'll make more money per customer and keep them in the store longer.
I remember shopping at Newbury Comics in 1984-5. They had music I could not get elsewhere - I was looking for German imports. Their ability to get stuff that nobody else had plus competitive prices enable them to grow. Do you have the stuff people want but can't get elsewhere? Coffee people can get most anywhere - or at least they think they can.
I take it back - the coffee market is so saturated, if you have a good location, people will come in looking for coffee. Otherwise, it is the books that will need to deliver the customers for you to shake down at the register.