Definitely read “The History of Middle-earth” series. The Book of Lost Tales is published as the first two volumes of “The History of Middle-earth.” It is like the OD&D of Middle-earth. It rocks—I consider it my single favorite book of all time. Keep in mind that Volume II contains all the most famous stories, so if you find Volume I dragging a little (though personally I love it), skip over to Volume II. So, give it a try. If you like it, you can probably keep going with the rest of “The History of Middle-earth.” Some of the middle volumes drag a bit, and a lot of the time you’re just reading drafts of The Silmarillion and The Lord of the Rings, but the first few and last few volumes of the series contain some really awesome works that are unlike anything you can find elsewhere. If you just want to know what some of those gems are, I can lay it out for you.
If you've read Unfinished Tales, History of Middle-Earth is similar, but with more versions of drafts and less relatively polished yet unfinished material. Unfinished Tales was sort of his "first scoop" through the unpublished material. It was published first and its success led to the Lost Tales and then the rest of the HOME series. I posted a list of some of the "gems" I found in HOME here.
Regarding the linguistic material (elvish language et al), I believe much of that has been published (with the permission of Christopher Tolkien) in journals such as Vinwar Tengwar: www.elvish.org/VT/
Last Edit: Jan 13, 2016 18:11:36 GMT -6 by Zenopus
tetramorph , if you haven't already barreled through everything, I would kind of suggest you dip into The History of Middle Earth volumes in accordance with your interests. Christopher Tolkien did an admirable job with this, as far as I can tell, but the nature of this material means some appeals a lot and some doesn't, at least at a given time.
If you're interested in the "Years of the Trees" narratives from the Silm, you may wish to check out The Book of Lost Tales 1 which has a lot of those stories in a tone strikingly different from that used in the published Silm.
If you're interested in the fates of Elves and Men, check out "Athrabeth Finrod ah Andreth" (a conversation between Finrod and a mortal) in the volume Morgoth's Ring.
If you have any interest in the "sundering of tongues" and how Elvish languages could drift so much although the Elves aren't in the habit of dying, check out "The Teachings of Pengoloð" in The Peoples of Middle-Earth.
Or look through the contents and select a few things that match your interests and then branch out. I don't know about you, but I would have been miserable and given up if I had tried to read these straight through.
Oh, I see Zenopus put up a list of gems, but I'll leave mine here, since they're pretty different.