As previously mentioned, if you are a Nebula voter, you can download an electronic reading copy of Mainspring [ Powell's | Amazon | Audible ] from the SFWA Web site here. Click through to the members link for the download.
Mainspring is available on Audible? How impressive! I love Audible - I've been a subscriber for about 6 years. It's even more awesome since adding iPod devices, so I'd recommend that anyone with an iPod look at this book!