Et dans lequel elle écrivait que

Question

Why is the imparfait used here and not the passé composé ? The author only wrote this once.

Céline Pickard · Accepted Answer

Bonjour Chris,
Here, the action is not seen as a one-time completed action (which would imply the use of Le Passé Composé) but rather as a description of what the book contains; hence, L'Imparfait is used instead. In other words, the focus is not on the punctual act of the writing but on the content of the book being presented as a backdrop.
EDIT: a hint has now been added in the exercise to avoid any confusion
I hope this is helpful.
Bonne journée !

Chris Wirt · Answer

The flavor of the French imperfect is like this:

dans lequel elle écrivait... -- In which she was writing...

With the passé composé it would be heard by French speakers like this:

dans lequel elle a écrit... -- in which she wrote...