Ivallan's Second-Hand & Exceptional Books
Ivallan's in Kreuzberg is one of the best and most interesting English bookstores in Berlin. I'm not just saying that because I had my last book launch there and they carry my books, although of course it's a bonus! (Another bonus is that it's around the corner from Pia, so you can get an iced latte afterwards). Even though it's small, they have a good selection of new and used books. The books are sorted into creative/obscure thematic categories and on random surfaces in shelves, drawers, suitcases, etc. so it's a fun place to browse, and you always stumble upon something unexpected.