This is the 3009th most frequent Russian word.
ею
The Russian word "ею" is the instrumental form of "она" (she), meaning "her" or "by her."
In this sentence, 'ею' refers to the previously mentioned book (in feminine grammar gender) and is used as an instrument or means in the sentence.
Она взяла её книгу и была довольна ею.
She took her book and was pleased with it.
Here, 'ею' represents the sunset (treated as feminine gender in the context), indicating the object of admiration.
Туристы любовались ею во время заката.
The tourists admired it during the sunset.
In this usage, 'ею' relates to a feminine subject implied earlier in the sentence, showing the doer of the action in the instrumental case.