I have songs that are representative of each manuscript, which top each hours-long writing playlist.
"Let Go" by Frou Frou, "The Con" by Tegan and Sara, "Amsterdam" by Peter Bjorn and John, and "Plowed" by Sponge for Book1, among many others.
"Golden" by My Morning Jacket, "Slow Show" by the National, "Fade Into You" by Mazzy Star, and "Something Pretty" by Patrick Park, etc., for Book2.
Sometimes, though, I'll get on a kick for a week with one song, and endlessly repeat it. It's like an addiction -- you know it's bad for you (because eventually you'll ruin the song for yourself), but it feels so good! A few recent obsessions have been "In Your Head" by Your Vegas, "This River Is Wild" by the Killers, and "With Me" by Sum 41.
Also, I have to second Sigur Ros. SUCH perfect writing music.