The Lost Road, the fifth book in the History of Middle Earth series, publishes for the first time the background material on Middle Earth J.R.R. Tolkien created for his own use as he wrote Lord of the Rings. Some of this draft material was edited and published posthumously in The Silmarillion. The Lost Road includes much of the material found in The Silmarillion in its previous incarnations, all with commentary from JRRT's son, Christopher Tolkien. Included as well is The Lost Road, a time-travel story Tolkien wrote at the behest of his good friend, C.S. Lewis (who was to work on a space-travel story). The Lost Road was never completed; Tolkien abandoned the idea midway.
Especially of interest to Tolkienian linguists are the Lhammas, or book of tongues, which outlines Tolkien's former conception of the dividing and multiplying of the Elvish languages. While this scenario is extremely out of sync with Tolkien's final conception, it is still of interest historically and important in order to understand the section that follows, the Etymologies.
The Etymologies are a /must/ for any Tolkienian linguist. They were and remain the greatest source of vocabulary in all the Elvish languages, and almost the only source for Primitive Elvish stems. While confusing to the non-technical reader, the Etymologies are still the main reference used by Tolkienian linguists. Allthough some of the material in the Etymologies is out of sync with what Tolkien imagined his languages to be as when he wrote Lord of the Rings, the changes necessary to bring Etymologies-style languages to 'modern' languages are mostly well documented.
In short, if you're just someone who enjoys Tolkien's works, this would be a fun read. If you're a serious Tolkienian linguist, this is a requirement.