This is the 8800th most frequent Russian word.
половина
Half.
In this sentence, 'половина' (half) refers to a fraction of a tangible object, the pie, indicating that half of it remains.
У нас осталось только половина пирога.
We only have half of the pie left.
Here, 'половина' is used to denote time, specifically 'half past' a given hour, indicating the time 9:30.
Он вернулся домой в половине десятого.
He returned home at half past nine.
In this context, 'половина' quantifies the portion of the book that has been read, implying that 50% of the book has been completed.