It does sound like it started a bit early, but it's not so uncommon as to be worrying!
In order to pick the right solution you have to have a good idea of the cause... different causes even have opposite solutions (e.g. persisting vs. backing off).
Some possible causes to investigate, including ones inspired by comments and other answers:
She feels tired. She could be physically tired from her day and need rest, or she could be feeling lethargic, e.g. from a long bus ride home, and need activity. (These two states often feel the same even though they have opposite causes and solutions!)
Her social battery is depleted and she needs time to recharge. Create space where she is left alone and not interrupted for a couple of hours (e.g. between getting home and having supper) and communicate this intention so she can feel secure that it's really her time.
She feels negatively towards the topic. No one likes dwelling on things they don't like, except once in a while to complain (if they feel like it and feel it would be received with sympathy). The solution here would be to (a) suss out what she doesn't like about her school life or reading and (b) try to help her change her situation, a whole other question.
She doesn't care about the topic. Similar to the above, but just the question needs changing, not the situation. Also, she could be preoccupied with something else. Maybe her whole school day is replaced by thoughts of her first crush or some friendship drama and it goes by without her noticing a thing she learned. If so, these are the things to be talked about instead (and, through discussion, brought into perspective so she can turn her attention back to daily functioning).
She needs to think about her answer. Maybe she has given very little thought to the things you ask about and feels put on the spot. Maybe she thinks there aren't many things you can say about how much you enjoy a character in a book. Some ways to address this: (1) Model answers, e.g. talk about the book you're reading in the same depth you expect her to talk about hers. (2) Give her time to prepare, e.g. announce that everyone will share something about their day at supper. (3) Ask more concrete questions, e.g. not did you like the character but did you think it was right or wrong when they lied to get out of trouble or whatever the plot point is.
She is beginning to form the idea of a private life. At each stage of gaining independence, we tend to overuse it in our excitement. A toddler learns the word "no" and starts rejecting everything because they can. A 17-year-old gets their driver's licence and starts coming home later than is good for them because they don't need to ask for a ride. A preteen realizes they have sensitive feelings they might want to keep to themselves and they start withholding even ordinary thoughts and opinions. If so, let her enjoy this but create lots of opportunities for sharing spontaneously (rather than when asked) and gently nudge the boundaries back to where they should be over time.
She doesn't like direct communication. This can be a symptom of the above but it can also be a developing character trait. Some people do feel much more comfortable side by side instead of face to face. Drive with her, walk with her, watch a show with her, play a game with her, do the grocery shopping with her, etc., so that she becomes more accustomed to your presence. Like a wild animal that has to understand you're not a danger, just being there calmly can create more trust and openness. (Yes, she's nine and has always lived with you... but in those early years a kid is practically a new person every couple of years and has to determine all over again who they are and what their role is in the family!)
She doesn't feel listened to. Maybe when she tells you these things, you don't show her that you actually care about her answer. Maybe you understand what she says but then expect her to do something contrary to what she told you. Maybe you forget what she said and ask her the same question the next day. (We're so much better at remembering what we've been asked than what we've asked others.) Maybe you just don't show in the moment that you understand and appreciate what she's shared. There's no shame in this; active listening is a hard skill to develop. Eye contact, not multitasking, echoing statements, asking pertinent follow-up questions, "Can you tell me more?", giving lots of time to think instead of speaking the moment you feel awkward, etc. are all good places to check whether you're showing her you care what she says.
She fears what you will think. This can come just as easily from your side or from her side. From your side, it could be explicit negative reactions ("Why did you do that? That was dumb!") or unconscious expressions of annoyance or disapproval (a frown, a sigh) or pointed questions that undermine her judgement ("How do you know he wasn't joking?") or signs of lack of interest or discomfort (changing the topic when she might have expected a sympathetic reaction). From her side, it could be a sense of shame about her general experience ("What if I didn't have a good day, when my parents are paying extra for 'forest school'? I can't tell them I didn't like it") or about something specific ("I definitely can't mention how I got in trouble"), or it could be fear of a wrong answer ("Am I supposed to like that character or not? If I say I like this trashy book will I have other books suggested to me?"). Either way, the solution is the same: a strong and consistent reinforcement of your warmth, your interest, your understanding, your approval, and your loving concern for things that are going wrong or actions you can't approve of, as well as modelling what it means to be vulnerable (maybe you didn't like school, maybe you got in trouble for something as a child).
So those are a few possible causes. How do you determine what the cause is in her case?
First, be attentive. Watch for signs of these things now that they're on your mind. Note that you don't have to be urgent about it. Spend a week and, with the other parent, agree to watch for one particular cause at a time. Decide whether that's the one.
Second, be direct. There is no harm, if you can do it lovingly and gently, in asking her directly: "Honey, when we want to talk with you and know how you're doing, you often give short answers or avoid answering. I would like to get to know you better but I'm not sure how to do that in a way that works for you. Can you help me understand what you're thinking?" If she's hesitant, you can start naming possible causes, again with no judgement attached, like I was doing above. Even just knowing that you consider it a reasonable thing to be happening in her head is a way of giving permission for her to acknowledge something she might not have volunteered.
Finally, you also have to be prepared for her carving out this space in her life not to share if she doesn't want to; that is, not to push if she really has decided not to share with you for now. But I agree it's too early to consider this a definite possibility.
I might update this answer if I think of more causes or strategies for finding them out, but hopefully this is a start!
