If you don't have a reservation (tables are booked more than a month in advance), know there are four barstools at the back of the restaurant that are first come, first served. It doesn't hurt to duck you head in the door and check.
Order a nice glass of Sancerre or a cold, crisp beer and order some bivalves of your choosing while sitting at the two-sided bar at the front of the restaurant. If oysters aren't your thing, there's a solid charcuterie board available for grazing.
There's beer on tap (a short list of local brews) and some great Spanish wines if you want to get started immediately, but the best thing about this coffee shop and book store is it's ability to slow things down.