I think when we went to Marga Schoeller most of the other customers were English-speaking - maybe I should mention the two rooms filled with English books? I should. And it's not like two rooms are so so much - If you live in Great Britain or Canada or in the USA you'll probably laugh at that. But it's two rooms filled with a really good selection of books. Also the people and the dog (Yes, they have a dog, called Ida.) are very nice and they can always recommend you some good books (Well, perhaps not the dog). And with the Germany children's books and young adult literature it's like with the English books: Not that many, but many good ones. Also the atmosphere is quite nice - it's warm and you feel comfortable around there.
The books aren't extremely cheap but I wouldn't call them expensive either.
I hope you enjoyed the second post in the "Bookshops"-series (You can find the information and the address at the bottom of the post).