Dan Roam's new book The Back of The Napkin is a hit. All in a style that is quick and enjoyable to read, Roam offers great practical advice that you can put to use immediately to solve problems and sell ideas with pictures ! He introduces a powerful system that eases the removal of the barriers and fears associated with visual thinking. I've really enjoyed this book.
Additional resources you may find useful:
A 53 minute podcast from the Principled Innovation Blog
Back of the Napkin Book Overview on Vizthink
A 59 minute podcast from Vizthink Podcast is broken into 3 segments.
Peter Durand's interview from the Center for Graphic Facilitation is also very good
The Back of the Napkin site and blog
More Napkin Sketch Examples on BusinessWeek

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