Nicholas Ganz, author of the amazing GRAFFITI WORLD (and others) is crafting a book on street art messages- with the goal to be "the first book, that will present the world-wide scene of artists, who exclusively work with texts, poems or slogans in public space. It will show the history of the written word in public space since the beginning of mankind and it’s importance until today."
Some wonderful artists are featured (myself included). Nicholas is working to raise money for it and has the German version of a Kickstarter page for it.
Check it out!