I'm totally a sucker for this kind of thing: Michael J. Trinklein's Lost States is a book about the dozens of proposals for statehood that didn't make it to reality. (Click on the cover to the left to enlarge, and you'll see some of the proposed states, including Acadia, Jefferson, Yazoo, Trans-Oconee, and Transylvania.)
Some of the what-if possibilities here are pretty astounding (did you know that parts of northern California and southern Oregon probably would have become the state of Jefferson if it weren't for the bombing of Pearl Harbor?) and Trinklein is an amiable-enough host, providing basic information in a clear voice about each of the potential states in alphabetical order.
The biggest problem I have with Lost States is that each almost-state gets a single page. In some cases, it's too little. In other cases, it's way too much. Just because a Senator suggests that England should become a state doesn't mean that it should get as much copy devoted to it as Jacinto, which has almost become a state on several occasions. I kind of wish that Trinklein hadn't decided on a photo-book format, with large illustrations of each almost-state on the page facing its description. I'd rather read a text-heavy, serious history book than this gloss over possibilities. Almost every state (including Washington) has had serious debate over splitting into two, and an exploration of this phenomena would make for some interesting reading that could maybe teach us something about wealth and demographic disparity in the U.S.
But as it is, Lost States is a quick little jaunt into the possibilities of what might have been had things gone in a slightly different direction. It's informative and fun, and if you're a history nerd, you should totally check it out.