Until the publication of The Children of Hurin a few years ago, Unfinished Tales was the third piece of the Middle-earth trilogy. Paired with The Lord of The Rings and The Silmarillion [confusion.cc] it was a bit of a third wheel; comprised of various stories in different states of completeness with some editorial comments by Christopher Tolkien needed to bridge the gaps and contradictions left by the myriad versions Tolkien authored of most of these stories.
Unfinished Tales actually includes my favorite of all of Tolkien’s writing. What I consider the be the best high fantasy ever: “Of Tuor and his Coming to Gondolin”. Versions of this story date back as far as 1911 and it is the birth of Middle Earth. It is an ultimate tragedy that Tolkien never finished the tale – “Of Tuor and The Fall of Gondolin” – to his satisfaction. I wish Christopher Tolkien could piece together a completed tale as he did with The Children of Hurin expanding the story in Unfinished Tales of “Narn i Hîn Húrin (The Tale of the Children of Húrin)” a related story to Tuor’s.
If you enjoy the window into Tolkien’s mind and the development of the Middle-earth “legendarium”. Then you can dive off the cliff into the massive 12 volume History of Middle Earth that Christopher Tolkien published in the late ’80s and early ’90s. An impressive read which I should attempt again as I’ve forgotten more of it than I remember.