I don't really know, I just know it was the process that was used for the UNC page replacement project when I worked on it after the flood. From what I can tell, books are flash frozen to prevent damage from ice crystals forming. Then they are dried in a vacuum to sublimate the moisture. It goes directly from a frozen to a gaseous state minimizing the water damage:

http://www.lib.rochester.edu/index.cfm?PAGE=3246

But really, I don't know how it works. But I do know there are companies that specialize in this.