That is a rather interesting point. My experience was as follows (I no longer mention things I've read in therapy):

1. Stuff related to understanding myself better (E.g. Harlow's monkey experiments, scapegoating, avoidant/fearful attachment type etc):
These were usually concepts the therapist had mentioned in the previous session. So, in my enthusiasm, I'd gone and read up a whole lot on it during the intervening week and was eager to discuss the insights I'd had (which were actually pretty helpful) in the next session.

I got a series of responses ranging from how I was constantly questioning the direction of therapy to being told I was trying to please her to trying too hard rather than letting things be (so, repeating patterns etc). But, basically no discussion at all happened and we moved on to other topics.

Honestly, none of those reasons were true for me (at least consciously) and I tried explaining but it didn't go anywhere and harping on it (given the $$ rate) didn't seem worth it ultimately.

2. Stuff not related to understanding myself better: This is just random stuff I've read (not mentioned by the therapist). When I referred to it though, I was told I was intellectualizing and so, repeatedly asked to focus on my emotions. But, again, no discussion etc.

So, it's been rather interesting for me to see the responses from others.