I had been looking for Codex Derynianus since the first edition was released in 1998. At that time it was a limited edition that quickly sold and second hand copies were extremely expensive and frequently over one hundred dollars. No library carried it. The book is some sort of guidebook / history / encyclopedia of the Eleven Kingdoms in Katherine Kurtz's Deryni Universe. It has been one of my favorite fantasy series for some time, though I far prefer the novels set earlier in the chronology. The world itself is ours, though a thousand years ago. It has the same religious backgrounds, but there is a race with the ability to do magic called the Deryni. The church has persecuted them mightily and there is much to do with the kingdom of a land called Gwynedd, which is located where our England is. It's a brutal fantasy series, but is fantastically written.
When Katherine Kurtz released a second edition of her Codex Derynianus I was finally able to locate a copy through my local library and I found out what the book really is. This is essentially an encyclopedia of her created world with entries on every single character that appears, every location mentioned or visited, and every kingdom and ruler. These entries are written in a form as if someone 1130 wrote the entries and his personal opinions (not those of Kurtz, but the fictional character) come through, especially when writing about the Deryni persecutions of the past and Hubert MacInnis.
There is also a timeline of the Eleven Kingdoms which gets into a good amount of detail when covering the events of the novels. It is this timeline that finally gives us hints of what went on in the year 948. I bring this up because that is the single year that has interested me the most in this series. It is 20 years after The Bastard Prince and is not covered in any novel, but Kurtz includes genealogies at the back of her novels and quite a few of the major characters presented in her books all die in 948. This is not likely a coincidence. Kurtz is at her best when she is the most brutal to her characters, so I imagine that when she gets around to writing the 948 book, it'll be a very good one. The timeline gives some coverage to what happened in 948, but I doubt that it tells the whole story.
So, Codex Derynianus is a good resource for those looking for background and reminders on the characters, events, and locations of the Deryni novels of Katherine Kurtz. It is clearly not a novel and thus not truly an exciting read. It's a fantasy resource for the work of one author. In that sense it is excellent, but for the average reader of fantasy I can't recommend it.