Vanished Kingdoms: The History of Half-Forgotten Europe by Norman Davies.
This large book (830 pp) was published by Allan Lane last year, and reviewed in the December 15, 2011 LRB. (Alas, the the entire review isn't available for view except for subscribers.)
If one is fascinated by places that carry allure in their very names -- Tolosa, Burgundia -(whose Burgundy, where and when?)! -- and, many a time, Poland but, and still, "Poland isn't finished yet," which can not be said for Ruthenia, or Tolosa.
Further, the LRB piece is a meditation in detail and depth on imperiums' inability to imagine their transience, and small, weak realms, who even more than comprehending how easily they can be disappeared, understand that their very languages are more than likely to disappear within another generation and a half.
It was reviewed again this week in the UK Guardian. Did come out in a paper edition? There's nothing to indicate why it was chosen to be reviewed in that publication now.
Another book that will be of great interest to fantasy writers who like to play Ruritania with European geography is the biography of travel writer, Patrick Fermor: Patrick Leigh Fermor: An Adventure by Artemis Cooper. Though the reviewer doesn't say this, Fermor's world has been as effectively disappeared by history as any invoked in Vanished Kingdoms. Fermor's time was a part of Merian C. Cooper's world, a world in which his explorations allowed him to imagine and create King Kong.