In a worldverse that the author, Marion Zimmer Bradley, famously refused to be constained by the details of other books and stories in the same world, this was the only sanctioned attempt at solidiying that universe into a consistent and coherent whole.
It was written by the MZB's husband at the time Walter Breen, who had been her husband during the publication of most of the Darkover novels (to that time).
While the concordance section of the book contains copius references as to where information came from in relation to a specific entry, not all the writing can be said to be non-fiction, a simple recitation of facts gleaned from other books. Some writing appears to come from knowledge of MZB's private notes, conversations with MZB, or were his own private extrapolations (that none-the-less passed MZB's scrutiny in greater or lesser degree).
Though Stormqueen! was published in 1978, it is not included. In the same vein The Bloody Sun underwent a major re-write that was published the same year as the Concordance. The Concordance is based on the original version published in 1964. (The re-write has some non-minor changes in Darkovan history).
An update to include the above and further novels was never published. In 1979, the year of publication, Walter Breen and MZB separated.