Polidor. It's on Rue Monsieur Le Prince. (Incidentally, the San Francisco Bookshop is in the same street - and you can usually pick up a copy of the Paris Walks leaflet there.) Nearest metro is Odeon.
Polidor is very French. It's been doing its thing forever. Wonderfully atmospheric. Red checked tablecloths.Old brass. House red served in bottles that are museum pieces. Very cheap. Very cheerful. It's always open. Packed out with a fun Left Bank crowd: Sorbonne students, arty types, journalists, savvy tourists, etc. And good hearty fare.
The world is a book and those who do not travel read only one page.