There is loads of material to draw from. The Unfinished Tales, the 13 tomes of the History of Middle Earth, some drafts that emerged later. The Lord of the Ring

There is loads of material to draw from. The Unfinished Tales, the 13 tomes of the History of Middle Earth, some drafts that emerged later. The Lord of the Rings is tiny in comparison. Some of the endings are missing, and some are only roughly sketched, but having the general outline of millennia of history really helps filling the blanks.