If my memory serves me well, there was a recent post, by
Geralt From Rivia, I think, that stated that DPC's previous system composed of four PCs with dual 3090s in each one.
I myself haven't played that scene where Quinn rejects the MC either. I have only seen one screenshot that
toobextra posted the other day and read the lines she says in that specific scenario. Anyway, to reply your question, she knows because she passes by when Jill is asking you to accompany her to her friends' dinner. It is my assumption that she got that info at that moment.
Besides the time when the MC hangs out with Quinn in episode 7 and asks her about that book, with her elusive reply, I recall other two instances in which we see it. One is that moment at the end of episode 7 you are referring to. If you pay close attention, you'll notice that she has a pen in her hand. Therefore, while it does not entirely rule out the possibility that she is reading (she might be indeed studying and marking important lines of the text), the fact that she glances a look at the top of the dresser or whatever that piece of furniture is, where we can see what is shown in the screenshot below, suggests that either it is indeed kind of her business book list or her diary.
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The other instance in which we get to see the book I was referring to is by the end of episode 8, just after the MC narrates how by that time he felt like, generally speaking, things were changing for the better. This is the screenshot.
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This image would reinforce the idea that in that book she keeps accounting or other business information.