1421: The Year China...
I purchased this as train-trip reading from Seattle to Memphis. It's not overly academic. The author makes some reasonable arguments to support his conclusion that there were maps of the New World before Columbus sailed. In fact, he makes pretty logical arguments that there were maps of the Straits of Magellan and several Caribbean islands before Columbus' voyages.
I recommend this for any social studies teacher, and for any upper-level/AP student.
Also available on PBS video, but I haven't seen it yet.