You can't do it in iTunes, but....
You can drag the audiobook from the iTunes books library to the Mac's desktop. Then you can use Quicktime 7 to play from 1/2 to 3X speed. Use the QT Window menu > Show A/V controls and use the playback speed slider at the bottom right of the A/V window. If you don't have QT7 player in your Applications/Utilities folder, this explains how to get it: http://support.apple.com/kb/HT3678