Via io9.com:
"Thanks to Windup Girl publisher Night Shade Books, people participating in this month's book club can write in to get a free epub file of the novel, which you can read on your computer and most eBook readers. (Fine print: You will be signed up for Night Shade Books' email newsletter when you get the free ebook - you can unsubscribe later if you don't like it.)"
More details here.
Windup Girl has been included on many "best books of 2009" lists, including Time magazine. If you'd rather pay for a copy in multiple formats, you can get it for $6 at webscriptions.com.