I have those as well! I found them hard to read the first time (many years ago). But when I tried them again after seeing the movie I found it a lot easier! I think you need to have a deep interest in JRRT's writing to take those on. I'm currently finishing up the History of Middle-Earth vol. 4, "The Shaping of Middle-Earth". Lots of interesting history showing how the mythology changed and developed.
The Appendices in Return of the King have a lot of interesting stuff as well, like the Tale of Years, explaining who had which ring, what happened to Aragorn and Arwen, Sam, Merry & Pippin, etc.