I love reading and although I also read German books and newspapers, it's always a joy to have some good books in English. There is actually a good selection at the bigger book stores in the Königstr.: Wittwer and Hugendubel. The first is just south of the Schlossplatz and the other on the first block north (towards Hauptbahnhof). The problem with these places is that they only sell new books which are not cheap. Nearby, across from the central University campus (not Vaihingen) at Schellingstr. 11 is the Piccadilly English shop which also has a book section. Warning - trying to access their website produces a malware advisory on my computer!
There are a number of places where on usually finds some cheaper used books: the OXfam shop in the Marienstr., which is a continuation of the Königstr. to the south. There is also the book store Pörksen at Schwabstr. 26 in Stuttgart-West that always has bins of used books out front for 1 or 2 euros. There are often some English ones but not a large selection.
There was an American who had a small shop in Degerloch with tens of thousands of books - all for 2 euros but unfortunately he died a couple of years back and the place is gone and sorely missed. This had been the ultimate source for used English books in the region for years.
Otherwise, flea markets are often the best place for used books. There is one coming up on Sept. 24th, 2016 at the Abenteurspielplatz, Schwabstr. 97, entrance from Forststr., which is just for books, CDs, records and games.
There is a huge flea market called the Pfennigbasar held in the Liederhalle on the western edge of Stuttgart center held by the German-American Women’s Club every autumn. It’s the biggest, oldest and cheapest ongoing flea market with lots of EVERYTHING including tons of books (nearly all in English) in addition to the mountains of cloths, toys etc. It often falls in October but is planned for the 8th and 9th of November, 2016.
Most all of the members of this club are the spouses of locally based American military personnel. Because of this association, there are usually some armed MPs running the security, a thing not normally seen outside off the bases. If I remember correctly, there is actually an entrance fee of a euro or 2 but one can find so much cheap stuff that it is definitely worth it. Visiting this event is one of the more off beat experiences in town! Proceeds from the flea market go to various social projects.
There is more information available on their homepage: http://www.gawc-stuttgart.org/
Also information at their Facebook page: https://www.facebook.com/pfennigbasarstuttgart