Very enjoyable for a maphead like me. It's really a series of discussions of the different ways in which mapheadedness manifests -- from a simple love of reading maps to orienteering, geocaching, and the Degree Confluence Project to creative map making. Jennings writing style is a little like a high school research report, with lots of footnotes linking to authorities and more information, and yet it's an easy reading style, relaxed and friendly. He includes lots of history of mapmaking, details of map design, and geography, but mostly as interesting asides. It's like a conversation with a particularly knowledgeable friend who loves to share his knowledge.
I recommend this strongly for anyone who likes maps at all.