I both love and hate the internet because you'll start in one place and ten links later you'll find you've gone down a rabbit hole and are looking at something completely unexpected.
I started reading an NYT article on where to eat and what museums to see in Berlin. A few clicks later I found these two interesting sites:
1. Berlin Wall Art. This site has a gallery of over 80 photos of the west side of the Berlin Wall, six months before it was torn down. Artists and visitors on the west side of the wall had the freedom of expression to draw and write messages on the concrete walls. I tried finding an art book on amazon that has a similar collection, but unfortunately nothing caught my fancy.
2. Banksy's Tour in Berlin. Banksy is a British street artist who does a lot of cool graffiti with stencils. This Google map shows 32 of his pieces artwork he spraypainted in 2003 and 2004 in Berlin. A few were featured in a book I have, Wall and Piece. Here are some good ones: