Thread: Have a ???
The History of Middle Earth
Letters of JRR Tolkien
Which I have..... plus there are more......
However, unless I was really an arch-fanatic or scholar, I would ignore the twelve volume H.O.M.E. series. For H.O.M.E., just like the electronic 'Hitchhikers Guide to the Galaxy', "contains much that is apocryphal or at least wildly inaccurate" in that it has contradictory story content that J.R.R. Tolkien never prioritized, but left to his son Christopher the educated guess-work of assembling these volumes: so I think he included it all.
The early HOME books are very early versions of stories which would evolve into the Silmarillion. Much within them has been superceeded, and as Grondy mentioned is wildly inaccurate to the finished product. Then follows the middle section of HOME books which are based around the writing of LotR. I haven't really delved into these properly yet, but again a lot appears to be changed (names for example... Strider is a Hobbit called Trotter, Frodo is Bingo etc).
The later HOME books, however, and by these I mean specifically Morgoth's Ring, The War of the Jewels and the Peoples of Middle Earth contain versions of stories Tolkien wrote after he had completed LotR. Again, they do differ from the published version of the Silmarillion, but the stories within them are greatly evolved, and the notes contain many interesting snippets written by JRR that Christopher omitted from the Sil. For those wishing to dig deeper than the Silmarillion or UT, I would recommend Morgoth's Ring.
Why have one book of the series but not the others? It doesn't make sense.
If you can afford to buy them all, or like myself, are lucky enough to have someone buy you the whole set for Christmas, then great, have them all. If, however, you can only afford one, or you only have time to read one, I think it makes perfect sense to actually choose the one you actually want. If you have read LotR, the Silmarillion and UT, it will make very little difference to you which order you actually read HOME in. They are not like novels that need to read in order. They are more like historic texts, showing the evolution of Tolkien's writings. Due to the fact that the later three books contain drafts that JRR wrote after he had finished writing LotR means the stories are closer to their final form, and also include material that never even made it into the Silmarillion. These three also contain material about Elven culture, languages and life/death/rebirth that you will not find anywhere else.
I still stick with my original advice. If you want to go further than UT, Morgoth's Ring would be my first choice. Don't let me put you off getting the whole collection though, Loni. Let me know when you have finished them.