I'm setting this thread up as a companion piece to the
Chapter-by-Chapter discussions now taking place in the sub-forum by that name. Some dedicated Tolkienophiles have said that, in addition to the LotR read-through, they will also try to do a read-through of the
History of Middle-earth volumes that deal with the writing of LotR:
Vol. 6 - The Return of the Shadow
Vol. 7 - The Treason of Isengard
Vol. 8 - The War of the Ring
Vol. 9 - Sauron Defeated: The End of the Third Age
Vol. 12 - The Peoples of Middle-earth
The Chapter-by-Chapter discussion welcomes appropriate side-references to HoME, but those who wish to go into greater depth may do so on this thread.