Paris has so many book stores but the one on most of the "Must See" lists is Shakespeare and Company. It is an english language bookstore opened in 1951 by George Whitman who encouraged young writers and let many sleep and work in the shop in exchange for working a few hours. Different owner today but the tradition has continued.
Peter has posted some information on the original Shakespeare's and photo of the site in his post on Hemingway.
We visited the Vanves Flea Market a week ago and there were many book dealers and many tempting old books with beautifully marbled covers and end papers. My friend spotted some she wanted to alter. I don't think I could do it. They are so lovely.