I never like when someone (other than in work contexts) is trying to force anything onto me, especially when they expect me to get into it (read, watch, listen etc) and inquire about it without my initiating the discussion or at least hinting that I am interested. Reading is definitely one of my favorite activities and I love discussing books, getting recommendations to assess for myself, I also often recommend reading to others. But I rarely loan actual books to someone without previously talking about it and knowing the person is interested, and vice versa... or at least knowing the person's taste pretty well and being confident it would be a good fit.

I think gifting can be different - you just give it and no need to ever bring up the subject again, unless desired. Loaning kinda implies the expectation of feedback and, without invitation, I find that intrusive. I am actually quite surprised that a T would just lend books to a client out of the blue. My last T loved reading and we had many overlapping interests in books (and art, and many other things), sometimes we discussed it briefly, but I did not want my therapy sessions to be about books or other things I routinely can and do discuss with pretty much anyone in my life. Sometimes we brought up things related to psychology, but only talked about it, never brought it into our meeting in material form.

If otherwise this is a good T for you, she was probably well-meaning... e.g. left the book to occupy you during the break, not thinking it would not be wanted. To me, it would also feel a bit like homework, but reading a popular fiction book that does not pique your interest is not really useful homework... or even appropriate, IMO.

In any case, I would be upfront with the T and tell her if you are not interested in those books - it can be done politely and I doubt she would be offended. I think it might be better to be direct about it also to avoid having to deal with this over and over again in the future. If you never say, she will never know and perhaps assume that you like it, especially if you are not truthful about your opinion and preferences. If you would still welcome theoretical book recommendations from her, maybe ask her to bring them up in conversation, perhaps with the possibility of borrowing the book ("There is this book.... in case you are interested, I own a copy and could lend it to you." - "Thanks, I may look into it."). And leave it there.