When people want to improve their craft, learn something or grow as a person they will most likely want to study and read. Depending what’s on their ‘read list’ you can tell what they’re interested in learning. This interest can give you insight in a person.
Not only is it telling for your interests, it can also tell you about how much follow-through you have or in what you’ve lost interest.
This does not mean that you need to read in order to improve, learn something or grow, it’s just a possible avenue.
Be careful
Don’t judge a book by its cover, or a person by what they wanted to read. People grow, might not have time or have a variety of interests. This is still just an indicator, it doesn’t tell the whole story.
Reference
You can gleam what kind of person someone is or wants to be depending on their anti-library.