I remember I loved the Fabled Lands British gamebook series (had to pay to have them shipped from overseas). Of course, I also remember that the series was canceled when it was still half incomplete.

So, what I want to know is: how much of the gamebook series was incorporated into this computer game? In particular, how much had the unpublished stuff been developed in the authors' notes and how is that incorporated into this game?

If you played all the books there will be nothing new for you beyond the changes that have been made compared to the books. (Mainly the battles but also some minor changes to events).

The base game contains the whole northern continent, that's books 1, 2, 4 and 5. I was somewhat annoyed that book 3 was not included.
The DLC adds book 6, but still not book 3.

I haven't kept up to date on the progress, but sometime last year the dev mentioned planning on releasing book 3 and 7 as more DLCs in the near future.

Great!!