The first forty-five minutes of Shopgirl were extremely irritating and it had everything to do with Jason Schwartzman's character, Jeremy. The greasy-hair, ADHD thing was fine in Spun, but I really didn't need another dose. Fortunately, it's at the 50 minute mark that Jeremy starts traveling with a band and the film shifts to focus on the developing relationship between Claire Danes and Steve Martin. Danes portrayal of a somewhat solitary woman who's stuck in just-a-little-bit of a rut is perfectly timed, Martin plays the wealthy but-even-more-stuck role in such a way that you never feel sympathy for him, and Schwartzman manages to style his hair and turn himself into an endearing, albeit somewhat kooky, kind of guy by the end of the film.
I've heard the book was considerably better, and a quick search on Amazon shows used copies for under a buck. I'd start there first.