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.

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