What exactly is the History of Middle Earth about?

[The Squatter of Amon Rûdh]

My favourite is The Lost Road and other writings (vol. 5), because it's very philological in its focus. It contains extracts from Tolkien's lectures on Beowulf, yet more Old English, early versions of Ainulindalë and some very interesting tie-ins with early Germanic legend; not to mention the fascinating early etymologies of Tolkien's languages.

I agree with davem about the content. There are points in the History of The Lord of the Rings where one's mind almost sinks under the weight of endlessly repeated drafts of the same passages; but Sharkû found out on which finger Frodo wore the One Ring from a footnote in HoME IX, so you can't afford to discount them. In my opinion HoME contains Tolkien's best writing; and Christopher Tolkien's commentary and notes provide a wealth of knowledge and understanding. The Lays of Beleriand contains some beautiful (and some rather imperfect) verse, and the gems that are C.S. Lewis' comments on The Lay of Leithian. All in all, it's difficult to sum the whole series up in few words, but certainly it represents an unique achievement. Almost everything that was Tolkien's writing is represented: poetry, prose, the baroque, the fantastic, the comic and the scholarly; and, of course, those sections in Old English (in one instance, Old English written in Tengwar). There's something there for everyone, but I doubt that everyone will want to read through the whole series. In my opinion it's worth the cost of the paperbacks just to find out which parts you like.