I enjoy map books quite a lot, and though it's a much different type of book than On the Map, my favorite map book that I've come across and which I read too far before starting IWMCW to review it here, I liked Lost States.
Lost States is an alphabetized collection of failed US states with interesting backstories. A lot of them occur in the 1800s, which isn't that surprising, but some of them were attempts made before independence, or as recent as the early 2000s. From Absaroka to Yucatan, Lost States gives details on bits of land that almost became US states, some of them bits of other, existing states, and some of them bits (or wholes) of other countries.