Tom
I know there is a quite good one on Boulevard Malesherbes. Think it is called Maison de Livres or something like that.
You can go to Metro Madeleine and ensure you face north (the street moves north-west). If you then walk north and use the left side of the street, you can't miss it. It is a bit to walk and the shop ain't that big, so keep an eye on the windows while you trundle along.
Last time I was there was in November last year. I walked to the place from Boule Mich so you can always comfort yourself with that I had a slightly longer walk...
If you have more time you can always go out to Musée de l'Air who has a reasonable bookshop department.
I also visited another place which I had been recommended by a friend. Unfortunately he just marked the place on my temporary map. Only open a couple of days a week during the afternoon. Very good, but I don't remember either name or address. Sorry about that.
Cheers
Stig