Middle-earth: Total War is based strictly on the lore and writings by JRR Tolkien and set at the end of the Third Age, at the time of The Lord of the Rings book(s).

The Lord of the Rings: Total War is based on the films by Peter Jackson so some elements are contradictory to Tolkien lore. This is set during the War of the Ring, just like the films.

The Fourth Age: Total War is based on the Fourth Age of Middle-earth. Although not based on any particular story by Tolkien, it draws its sources strictly from the writings of JRR Tolkien.

LotR:TW can be reached from the links in my sig (or ByzantineKnight's)
MeTW and FATW's forums can be found at The Chamber of Records.