"The Dictionary of Imaginary Places" by Alberto Manguel is literally a gazetteer of imaginary places. I read the old version 25 years ago but its been updated to include places from newer works like Hogworts and Jurassic Park. Its entries aren't single sentence descriptions but are often pages long with maps and what you would want to know as a traveller.
http://www.amazon.com/Dictionary-Ima.../dp/0156008726
Google Books has a preview:
http://books.google.com/books?id=T36...age&q=&f=false