This book is significant as being the first collection of mostly-unpublished JRR Tolkien writings to emerge since the death of his son Christopher; being edited by Carl Hostetter. It is a book that serious Tolkien readers will want to own and use; because it is something like a cross between The History of Middle Earth Volume XII and an addition to Unfinished Tales.
Therefore, those who regard HoME and/ or UT as important, will not want to miss this one: there are many things here which I regard of permanent value; and I am very grateful to know them.
These are too many to list. Perhaps the essay Osanwe-kenta may take pride of place (I had already read this, but it was previously only available in a small circulation magazine). This is a detailed consideration (relevant to both Middle earth and our own world) of the 'communication of thought' by what might be termed telepathy, including the implications for free will. This was of extraordinary depth and interest to me.
Another section is given the title of Description of Characters, and is Tolkien's commentary on Pauline Baynes's illustrated map of Middle Earth from 1970. For example, Tolkien regarded the back-view of Boromir as an almost perfect representation of his imagination; while Sam and Gimli were 'good enough'. On the other hand he was critical of several (most!) others including Gandalf, Legolas, Aragorn, Shelob and the Black Riders; and in making these criticisms he gives further detailed accounts of how he saw them.
The bulk of the book is about elves; including a great deal about their different types (also why they ended up different) and very large scale considerations of the justice and wisdom of the Valar in their dealings with elves. All this was absolutely gripping, and operates at a very deep spiritual level.
The book does, however, have a significant flaw in that it begins badly - i.e. boringly. There are 166 pages under the section title of Time and Ageing - with repeated developing drafts of Tolkien's consideration of the time-lines, time-experiences (relative between the undying lands and Middle earth, and between races), movements of elves; and a lot about the elves rates of growth and reproduction. These are often presented in terms of tables of figures, and lists.
There was a great deal to interest me among these 166 pages; but there is no denying that there is a great deal of repetition and minutiae. There is considerable relevance - even here - for those of us fascinated by Tolkien's mind and creative processes. But I think it was - I believe - an error to begin the book with this material; because some readers who open and start reading at page one, will probably get no further. I would have placed this material in the middle of the volume; and probably relegated some of the repetition and tables to an optional appendix.
The last section of the book, entitled The world, its lands, and their inhabitants - may have the broadest appeal to those whose interest is focused on Lord of the Rings. It consists of 100 pages of extra 'snippets' about all kinds of things. I was particularly pleased to learn more about Numenor (everyday life, reproductive life, ageing etc).
In general; this volume is (I think) the first to provide abundant new authorial material on what might be termed demographic, social and organizational aspects of Middle Earth (and the undying lands) - which are often stated to be neglected by Tolkien because they are omitted from Lord of the Rings. The Nature of Middle Earth proves that Tolkien had indeed considered such matters, often in considerable detail - albeit often after the publication of LotR.
Overall, this is a book aimed at Tolkien aficionados such as myself; but for us it takes its place alongside John D Rateliff's two volume History of The Hobbit - valuably supplementary to the many earlier (and uniquely, filially, authoritative) volumes edited by Christopher Tolkien.