I believe I can help you with your concern
I can understand your concern about finding this content on her Kindle. May I ask, what exactly are you trying to find out by asking more questions? If she wants a lesbian relationship with someone, may have been intimate with a woman in the past or recently, thinks she may not be heterosexual anymore, etc..?
Okay, well that makes sense then. Well first you can her how long she has had fantasies about women and to what extent (erotica, companionship, relationship, etc...)
Has she ever been with a woman before? Was this before meeting you or after meeting you?
Is she still attracted to you would be another good question?
Also does she masturbate to this literature or fantasies? That would judge the extent of her fantasies and how much she is infatuated with them
Another question to ask, but I would do it later it in the conversation and if she has expressed interest in developing a sexual relationship with a woman would be to ask if she wanted to add a woman in your sex life with your wife
It is uncommon to be honest, but many women out there are just attracted to the female body and lesbian erotic literature, but do not act on it or want to in real life.
They are mostly just fascinated and admire the lesbian erotica and female form
It would not hurt going to couple's therapy to explore the extent of this as well
Well I think usually these thoughts develop when you are a young adult, but if you do not act on them or experiment at that time, then by the time she hit middle-age she started to get a desire to explore that before it was too late and this is why it may have triggered this compulsion that she is talking about.
Well i would talk to her first and see if this is very ingrained and these interests is lesbian erotica are at a point where she wants to act on them. If they are, then I would suggest couple's therapy to discuss why these interests popped up and what she wants to accomplish with them. It is possible couple's therapy may establish that this is just a phase and it will pass