In the Eastern side of Berlin, close to Eberwalderstrasse and Greifswalderstrasse, at Saint Georges, I discovered two big rooms, stuffed with English books. Used books, but not second-hand. You could find there good books, 1-2-year old, at 4-5 Euro the book, but also new ones, for 10. Many many of them are dedicated to Germany's history.
The bookstore is also offering the possibility of ordering books, or if you have at minimum 15 and high-quality titles for sale, they could visit you and offer a proper financial evaluation of your collection. A fair deal, I think.
And, of course, everybody is speaking English there.