Borchsenius had never worked with Bournonville, but had made notes from working with Beck (the first great Bournonville stager). She kept them in a sewing basket -- just little pieces of paper. During rehearsals with Lander, she would rummage around and pull out a note or two to make a correction. Lander urged her to write them down, and she did. So "The Notebooks" are 50 years post-Bournonville, and Hans Brenaa would sometimes put in something that he remembered from his childhood that wasn't in the notebooks -- always a tiny detail, usually mime.
If you're interested in the history of Bournonville stagings, may I humbly refer you to my articles about this that are now available on line:
A series of three articles outlining the posthumous performance history of Bournonville's ballets in Denmark from 1879 to 1992, the tragic events that befell those works after that time, their current condition, and a gloomy prognosis for their future. These articles were originally published in DanceView magazine. A shortened, two-part version was published in the British quarterly Dance Now, under the titles "Bournonville in Hell" (Spring 1998) and "The Mermaid's Head" (Summer 1998).
I should also add that there really isn't much about Bournonville style, per se, in the Kronstam biography, although you will be able to get information about it from comments the dancers make, I think. It's more about the tradition generally -- the training, the school in its broadest sense.