Nine volumes? What's been going on besides all that ring stuff? Anyway, sounds like it could have been a ripping good yarn.
Most of The History of Middle-earth's 12 volumes deals with Tolkien's wrestlings with his First Age materials. Three of the volumes chronicle the composition of The Lord of the Rings. Two unfinished novels, the Lost Road and The Notion Club Papers, are provided. Late volumes in the set have some interesting musings by Tolkien on LotRian matters such as the origin of the Orcs.
Christopher Tolkien had mixed feelings about the book published as The Silmarillion, which was his effort of prepare a readable presentation of his father's First Age stories, and, many of us would say, a successful effort. However, Christopher soon released Unfinished Tales, which helped us to see better the complexity of Tolkien's imaginative work; and then we were given the HoME. These are a noble monument of filial and scholarly dedication, and I am sorry for people who say they think they are cynical efforts to milk the Tolkienian cow.
I recommend a look at the early essays in this book
Tolkien's Legendarium - Tolkien Gateway
as something of a guide to the HoME volumes. But better for many will be just to read materials of interest. For me two of the highlights are
The Notion Club Papers in Vol. 9 (Sauron Defeated) and
Athrabeth Finrod ah Andreth in Vol. 10 (Morgoth's Ring). The latter is probably the great "unknown gem" by Tolkien.
Athrabeth Finrod ah Andreth - Tolkien Gateway
Tolkien's brief attempt at a sequel to LotR in Vol. 12 (The Peoples of Middle-earth) is at least an interesting curiosity.
The New Shadow - Tolkien Gateway
Well worth reading is
The Lost Road, an unfinished novel in Vol. 5 (The Lost Road and Other Writings).
The Lost Road - Tolkien Gateway