Gary, I am interested in getting the Learth Gazetteer:
http://index.rpg.net/display-entry.phtml?mainid=7239
Was it published in book form, or only in pdf? I'm having a bit more trouble finding a copy than I thought I would.
Also, how close is "too close" for you for a fantasy Earth setting? Your Aerth is pretty darn close to the actual Earth. How close could it get? What about (for example) an Earth with geography and history virtually identical to our own, set in circa A. D. 1500, BUT with no gunpowder and with only historically insignificant magic and monsters? By "historically insignificant" I mean that such things tend to be in far, out-of-the-way places, and tend to be encountered only by adventurers specifically looking for them. A fantastical dungeon, for example, filled with all sorts of D&D goodness would fit in the campaign. A horde of gnolls besieging Constantinople, however, would not. The former would be able to escape the historian's eye, while the latter could not.