I think that it's more interesting to read a book in English, that you havn't read in German before.
But i think this opinion can be different from person to person.
I prefer to read a Book in English... at the moment i'm reading the first Book of Harry Potter, because i have to improve my English for my future Job and i think that this is a good way (besides others) to get back into the English language.
But i also think that you have to talk and to write a lot of English to get more experience in the active use of this language. Reading should be only one part of your plan.
EDIT:
One additional question: Does anybody hear one of the Harry Potter Audiobooks ? I would like to hear some experiences.... do you learn english better if you listen to it ?