There is another small bookstore at/in the OlympiaDorf that sells used English books - along with used German books. And cheap - less than 1€.
I'm guessing the address is Helene Meyer Ring, but, here's how you get there:
Take the U3 to OlympiaZentrum. Start walking towards the Olympia Dorf(where all the athletes lived during the 1972 Olympics). Walk away from all the BMW buildings. Walk up the ramp, past the Post Office, Greek Resturarant, and Magic Hair on right, Wimmer is on left. Turn right at the Bank(right after Magic Hair). Pass the bank and a camera shop and it's right there on your right. If you get to
Tengelmann, you've gone too far. So, a 5 minute walk from the Olympia Zentrum.