In content the book is entirely narrative (or descriptive).
The narratives in this book are indeed on an altogether different footing: taken together they constitute no whole, and the book is no more than a collection of writings, disparate in form, intent, finish, and date of composition, concerned with Númenor and Middle-earth.
The narratives are all "unfinished", but to a greater or lesser degree, and in different senses of the word.
A simple structure has been applied of dividing the texts into Parts corresponding to the first Three Ages of the World.
The Fourth Part is an appendage, and it contains generalised and discursive essays with little or no element of "story."