The Fall of Arthur:

The Fall of Arthur By: J.R.R. Tolkien ed. by: Christopher Tolkien
Fiction: Poetry, E-book

This book is Tolkien's version of the tales of King Arthur, and his son's commentary and notes on the book.  The actual story of King Arthur is only about a quarter of the book.  Most of it is note and commentary.  Some of it is interesting to see how the story connects with the tales of Middle Earth, or how Tolkien worked on things.