I saw it in
The Atlas of Middle Earth. And as Lindolirian said, it shows Mordor was right smack in the middle of the Inner Sea of Helcar during the First Age.
(Shameless plug for one of his favorite Middle Earth books). I've found the Atlas to be a really useful thing to have around. Not only does it have good maps but some pretty good summaries and analysis of events. Well, for the most part. I don't entirely agree with all of Fonstad's stuff, but on the whole it's a very good work.
*SIGH* Sad to say I don't have
The Shaping of Middle Earth, being cash poor and all that.