Steve Martin did write Shopgirl and both the movie and the book are amazing. Sophia does only have two movies out but her newest movie Marie Antoinette comes out in October and looks unbelievable. Kirsten Dunst and Jason Shwartzman. Need I say more?
Under most circumstances I would definately say read the book first but they are both so similar it makes no difference. I have the book and love it but saw the movie first. I AM IN LOVE WITH THE MOVIE! It comes out on DVD April 25th. It was one of my favorites from last year. If I find another used copy of the book Ill mail it to you.