The offer is open to the general public since the public has access to the RootsTech expo hall. Sign up, drop off the book at the scanning booth in the morning, and pick it up at the end of the day along with your digital copy.
The book must meet the same guidelines as donations to the Family History Library. You must have a signed permission form from the copyright owner, or you must be the copyright owner, or the book must have been published prior to 1924 (presumably in the United States). Check the existing FamilySearch online book collection to see if they already have the book.
To allow more people to take part in this offer, FamilySearch is accepting books no longer than 400 pages. That was killer for me. The only qualifying books I own are too long. (I wonder what they would do if I signed up for two books and showed up with one book of 500 pages?)
Hurry! Slots are limited. Read more and sign up at http://rootstechbookscanning.eventbrite.com/.