Welcome to HealingWell. I am glad you want to understand your partners depression. This is truly a difficult disorder to live with but most people work with thier physicians and therapist can live a very nornal life.
The most popluar book right now appears to be Understanding Depression By Patricia Ainsworth.
Another place to look for resource books is on HW.......to the right you will see a green box that says Resources. There is a lot of material available.
It sounds to me like your partner is feeling insecure and depressed right now. That could be why she keeps asking you the same question, not that she does not trust you. She is looking for validation that you care. Depression is frightening. Many people with depression are afraid of abandonment.
I hope this helps you in some small way.
Take care and keep posting.