This book may not be the most enjoyable you've ever read, but it is the offical guide put out by the ICC. It is broken down by Incoterm and into each category of obligation by the seller and buyer. This is the book that will be translated into multiple languages for everyone in the world to read. You really can't ask for more. However, if you are new to Incoterms, don't expect to understand it after going through it only once. It's still not an easy subject to grasp for new exporters/importers.
For American exporters/importers I would definately recommend reading a supplementary book put out specifically for Americans. Incoterms are partially written with European countries in mind, so there are important considerations for countries with borders and exports laws similar to the USA. The best supplimentary book I've seen so far is "Incoterms for Americans" by Frank Reynolds, the USA representative to the Incoterms 2000 revision meeting. The 2000 version may not be officially out yet. I got it at a seminar.