Ask any writer: the two most difficult parts of the writing process are 1) sitting down in the seat to write and 2) figuring out what to write about. We can't help you focus on your work, but we can try to help you find inspiration in the city around you. That's what Seattle Writing Prompts is all about. These prompts are offered free to anyone who needs them; all the Seattle Review of Books asks in return is that you thank us in the acknowledgements when you turn it into a book.
I haven't read that many good books about graffiti. Jonathan Lethem's Fortress of Solitude is the first one that comes to mind, though I know I've read others. Why isn't there more good fiction about graffiti? Don't writers love to write about writers? And aren't graffiti artists just another kind of writer, explaining the city around them?
If you're having trouble finding inspiration, go take a walk. Focus on noticing the graffiti. When you find a piece that speaks to you — maybe it's half-finished, maybe it's clever, maybe it's crude — try to write about it. What was the artist trying to say? What are the different ways the piece could be interpreted? Where's the artist right now?
If you're unable to go out for a walk, my friend Renée Krulich has a world-class Flickr stream on which she documents seemingly every single piece of graffiti in Seattle that she can find. They're all organized in albums alphabetically by artist, from famous names like John Criscitello to sticker artists like 'Phones. Personally, I'm fond of Sti(c)kman and Allo, but you can choose whomever you like. Or maybe you finally want to get to the bottom of the eternal Seattle question: what's the deal with SHITbARF? There must be a novel there. At least.