Gibert Joseph, both new and used books (mostly French) are on maximum display here.
26, boulevard Saint-Michel.

Shakespeare and Company, the most famous of English bookstores in Paris.
37 rue de la Bucherie near métro Saint-Michel.

Abbey Bookshop, secondhand English bookshop.
29, rue de la Parcheminerie, Metro/RER: St-Michel.

Seine-Side Traditional Outdoor Booksellers.
Most booksellers are open daily from around 11:30 am to sundown, and close during national holidays and in the case of heavy rain or storm conditions.
Left-bank locations: Seine-side from Quai de la Tournelle (Metro Maubert-Mutualité) to Quai Voltaire (Metro Saint-Germain-des-Prés).