Les Bouquinistes is one of my favorite restaurants. It is always superb and I would place it high on my short list.
For lunch, at far lower cost and not too far away from les Bouquinistes, you might consider au Babylone on rue Babylone. The restaurant has been operated by the same family, at this same location for many years. It has old world charm and you´ll find few tourists dinning here. Arrive early for the best selection as when the menu specials are gone, they are gone.
Another favorite place for lunch is le Petit Vendôme, 8 rue des Capucines in the 2nd. They feature delicious pork dishes and charcuterie, the service is friendly, unusually so for France, and the prices are very modest. It is always packed for lunch but if you catch the waitress´s attention, she will find a place for you.
Another interesting location for lunch is the counter at l´Avant Comptoir, part of the Hotel Relais Saint German at 9, Carrefour de l´Odeon in the 6th. Stand at the counter and order delicious sandwiches or unusual entrées such as œuf cocotte or hareng a la crème. Salted butter from Bretagne, dark bread, or cornichon are available to accompany anything you might order. They also have a great selection of wine by the glass and an informed staff that can help you chose an appropriate glass.