This document is more general than my other document, which is specifically the commentary on the history of The Lord of the Rings; in time I will possibly have a document on The History of The Hobbit. There aren't nearly as many sections as there are in the history of The Lord of the Rings document but there still a few with some lesser known things including a little known jest to do with Hobbits that Tolkien elaborates on in the appendix on languages. All of these can be found in different parts of The History of Middle-earth [HoMe], Unfinished Tales [UT], The Letters of J.R.R. Tolkien, The Silmarillion [TS], The Hobbit [TH] and The Lord of the Rings (LR; The Fellowship of the Ring, FR; The Two Towers, TT; The Return of the King, RK).
You can skip to the abbreviations in this document here, the vocabulary here, the notes on the structure of this document here and to the first section here.
I only recall abbreviations for the volumes of History of Middle-earth for The Lord of the Rings books as far as the History but the abbreviations that I do know and that are (or might be in a future update) are below.
As for how I've gone about reading the history it's important to realise there is a lot of background information, a lot of history with many drafts, ideas changing and essentially everything being different in some way or another. I started out initially reading from the first book but then I decided that I wanted to read the history of The Lord of the Rings first; therefore I started with book VI, entitled: The Return of the Shadow (a name that Tolkien thought of before deciding on The Fellowship of the Ring). I have since then read more of HoMe and along with UT and various other works I have the content in this document (as in these are the primary sources here).
I will try my best to organise this in a somewhat intuitive order where possible (and when I have the motivation to do so) but I personally prefer having more content over having the best organisation. But just like Middle-earth there will be many drafts and versions of this document; I will try to document where possible but this will never be remotely perfect.
Those who have read TS (or the story that was published 2018 on its own) will know about The Fall of Gondolin; in this tale Glorfindel slays a Balrog but like everyone else who does he falls with it. Now Elves are by natural law to return to the Valinor when their body is destroyed and to await judgement. Those who have done real evil are judged by Eru himself and perhaps the most significant example is Fëanor (who is not reincarnated).
Glorfindel, however, was forgiven for his part in ignoring the Ban of the Valar (see TS) in the Kinslaying &c instigated by Morgoth (he corrupted the Noldor or at least some of them and when he stole the Silmaril Fëanor sought revenge and he and others made a vow to reclaim them and they murdered other Elves who refused to give them their ships to sail to Middle-earth). This is because he actually was against the Kinslaying and he only joined out of loyalty for his King (as I think it was - can't recall off hand).
At this point because his home of Gondolin was destroyed he stayed in Valinor for quite some time; here he would befriend Olórin (Gandalf), a Maia, and having done this, was almost as powerful as the Maiar themselves in spirit. This is why the Witch-king of Angmar even fled from him (this is in the Appendix - I want to say Appendix A). Now it's said that he could not deal with all of the Nine Nazgûl at once but as we know in the FR their horses were driven mad by the water and along with the fire from Aragorn and Glorfindel along with the latter's wrath they all were dismayed.
Glorfindel of course is the one who rescues Frodo after he had been stabbed at Weathertop by the Witch-king, Lord of the Nazgûl. The last time we see him is in The Council of Elrond but either way he was a very powerful Elf.
There is a lot more to say about Balrogs but here are a few comments. What is a Balrog? As I noted in the vocabulary they were servants of Morgoth but then what does it mean? Well at least one 'translation' of the Balrog is a 'demon of terror'. They were Morgoth's most formidable servant and it was they who rescued Morgoth from Ungoliante (at other times spelt Ungoliant) after he refused to hand over the Silmarils (he did give other jewels but she was greedy); they went after her as she fled but Morgoth called them off. She would later have an infamous offspring: Shelob.
There is also a very contentious debate about whether they can fly; I have written an essay on this with a different angle here and there is another good discussion at The Encyclopaedia of Arda. This debate largely derives from way Tolkien worded the encounter of the Balrog called 'Durin's Bane' with Gandalf as well as the false belief (and this is actually noted by Gandalf in The Two Towers in the chapter 'The White Rider') that the Winged Messenger was the Balrog. Gimli believed it was but Frodo knew otherwise but he did not want to say: for the Morgul-knife did have a huge affect on him. But there were other signs that it was indeed a Nazgûl besides those facts. In fact they had already had a flying mount for some time: the first time they encounter one (though it flies over them and does not swoop down on the Fellowship) when the Fellowship is struggling with the snow on the mountain prior to the decision to go to Moria which of course is where the Fellowship encounter the Balrog. This Balrog had fled during the First Age during the 'War of the Wrath' until the Dwarves delved too deeply in Moria.
Finally for the time being at least, as I have noted, they play a more significant role in The First Age and one such place is The Fall of Gondolin where two are slain; both Balrogs took down their opponent and one happened to be Glorfindel who is the same Elf as in The Fellowship.
As I noted in the vocabulary Tolkien noted in more than one place that the plural of 'dwarf' should be 'dwarrows'. He pointed out in the Letters that it was a bad habit of his to use 'Dwarves' and if I recall correctly he did this to make it like Elves. This was something that publishers gave him trouble with but even worse was the American publisher that tried changing 'elves' to 'elfs' which he rightfully rejected utterly (since it's never been that nor indeed is it a word).
In the second chapter of The Peoples of Middle-earth 'Appendix on 'The Appendix on Languages' Tolkien elaborated on the plural a fair bit. He noted that he didn't know at the time he wrote Dwarves that it should be Dwarrows rather than 'Dwarfs' (see below) but he did refer to it in one place in LR; The Mines of Moria (Moria in Elvish means 'Black Chasm') is in the Common Speech Dwarrow-delf 'Dwarf-delving'.
He notes that dictionaries would have us believe it 'dwarfs' but he explains how and why it should be dwarrows but I feel that that's of less interest here compared to the above.
In the Appendix on Language in HoMe XII, ‘The Peoples of Middle-earth’, Christopher cites his father on many things that would otherwise be unknown. There has been the suggestion that hobbit was a word that Tolkien might have seen elsewhere but this is unlikely and in any event unconfirmed. But Tolkien went much further. A while back I noted to a long time friend that there is an interesting thing about Hobbits and - rabbits! There is much more to this including some issues of translation that he did not foresee but (as expected) he brilliantly solved but this is the information about rabbits. This is what Tolkien had to say (at least as far as rabbits goes):
This, I confess, is my own invention; but not one devised at random. This is its origin. It is, for one thing, not wholly unlike the actual word in the Shire, which was cūbuc (plural cūbugin).*
But this cūbuc was not a word of general use in the Common Speech and required an equivalent that though natural enough in an English context did not actually occur in standard English. [...]
* For another, I must admit that its faint suggestion of rabbit appealed to me. Not that hobbits at all resembled rabbits, unless it be in burrowing. Still, a jest is a jest as all cūbugin will allow, and after all it does so happen that the coney (well-known in the Shire if not in ancient England) was called tapuc, a name recalling cūbuc, if not so clearly as hobbits recalls rabbit. [This note was later struck out.]