Karen Wynn Fonstad compiled an Atlas of Middle-Earth.
She studied the lands and geography of Arda across the 1st, 2nd and 3rd Ages. I have found it to be an excellent guide for role-playing, as it includes maps of lands, journies of key characters in each age, diagrams of wars and war fronts, military lines and strategems, guides on topography, inhabitants, plants and animals lives, climates, weather patterns and even how far Tolkien had people travelling by horse on on foot by Race of character (Elf, Dwarf, Man).
I would have to say I have no quibble with it's accuracy from what I have observed, but like all guides it is not direct source material and so possibly open to some dispute over it's veracity.
Hopefully I've got a hold of the right end of the stick in looking at what she listed for the 3rd Age. Reviewing that section, here are what the Atlas has by way of maps:
- Middle-Earth: including the realms of Forodwaith, Eriador, Arnor, Rhovanion, Rhun, Gondor, Mordor, Khand, Haradwaith
- The sunken peak of Memeltarma, the very tip of which seems to appear as a small land mass in "The Sea"
- The outline of the sunken Numenor, now re-named as Atalante ("The Downfallen")
- The New Lands, which I group as "The West" and would inculde Tol Eressea and Aman
Interestingly though, and hopefully not to only me [img]smilies/wink.gif[/img], there is a large map of Arda as at the 1st Age. In addition to the lands of Middle-earth and Aman (including Tol Eressea) there are other lands named:
- Hither Lands
- Dark Lands (South Land)
- Walls of the Sun (Kalorme)
Looking at the map, it is conceivable that perhaps all these lands were seen as approximations of actual continents. Tolkien was creating am ancient mythology for his Britian in his writing and it has been suggested that his lands are loosely based on ancient earth lands, similar but different on account of things like continental drift and the fact that his "mythology" does not need to be based on actual continents. It is not factual.
So he started out Arda's history with more lands. Yet, they are not mentioned in the Third Age. Whether they fell in the consequence of the terrible Wars of Power and Wrath in the First Age where the Valar widely exercised their astonishing and vast powers against Melkor, or whether they simply weren't relevant to the stories unfolding in The Hobbit and Lord of the Rings is a whole other question.
I'm inclined to rest my answers not on what is in the History of Middle-earth series, but what JRR Tolkien directly related in The Silmarillion, Hobbit and Lord of the Rings. It is from this that the Atlas I have referred to draws.
Hope that helps.