Learn Spanish (Latin America): La chica que leía el libro era madura, pero estaba delicada. - The girl who was reading the book was mature, but she was frail.

Using ser (here in the past as era) with madura describes someone's character as mature, sensible, or grown-up. If we used estar, it usually describes a fruit as being ripe and ready to eat. Since we are describing her personality, era is the correct choice.

How does the meaning of delicada change because it uses estaba instead of era?

When you use estar with delicado or delicada, it means the person is currently frail, sickly, or in poor health. If the sentence had used era delicada (using ser), it would mean she was a sensitive, fragile, or fussy person by nature.

Why are all the verbs in this sentence (leía, era, estaba) in the imperfect tense?

The imperfect tense is used here to set the scene and describe ongoing background conditions in the past. We are describing what the girl was doing (leía), her inherent personality trait (era), and her physical state at the time (estaba), without focusing on when these things started or ended.

Why does the sentence use que for who instead of quien?