Rateliff takes over as the chronicler, doing an excellent job of examining the evolution of The Hobbit, and how it tied into the emerging legendarium. Christopher Tolkien basically skipped over The Hobbit in the main history. If you make it through The History of Middle-earth, you might as well read The History of the Hobbit. If you do, consider replacing Volume 9, Sauron Defeated with The End of the Third Age, which includes only the parts of that book that deal with The Lord of the Rings.) The Nature of Middle-Earth could be considered a 13th volume of the History, assembling some works published elsewhere with a series of other notes and variants. (If you only care about The Lord of the Rings, you could just read volumes 6-9. The History should be read in volume order, as later volumes make references to earlier ones. Expect large chunks of any story you might get into reading to be removed, and replaced with a reference to The Silmarillion, The Unfinished Tales, or an earlier volume of the series. If you do read it, you will want to use two bookmarks, one for the primary text, and one for the copious footnotes that follow. It is not a series of stories, but an extended discussion of the writing of The Silmarillion, The Lord of the Rings, and some ancillary works. You should not read The History of Middle-Earth unless you are totally fanatic. The Letters of Tolkien is a very interesting read, and could really be read almost anywhere in series. Familiarity with The Silmarillion is essential here. They contain some of Christopher Tolkien's notes about the source of the text, but are mostly readable as stories. The Unfinished Tales are exactly what it says on the tin. These books are written to present that material in a smoother, more entertainment oriented fashion, rather than the dryer and more academic tome of the other versions. All three of these contain significant amounts of re-printed text from The Unfinished Tales and The History of Middle-earth. Beren and Lúthien and The Fall of Gondolin are collections of the varying versions of those tales, stitched together in a directly readable order.

The Children of Húrin is a complete and detailed version of that story.

They require an understanding of the First Age stories to appreciate. The Great Tales trilogy are stories from The Silmarillion in more detailed forms.

The Silmarillion does not presume that you have read The Lord of the Rings, but would probably not be interesting to someone who is not already invested in Middle-earth. (You may very well be able to get away without reading it, but there are clear back-references.) The Lord of the Rings presumes that you have read The Hobbit.

The silmarillion by jrr tolkien