According to the Oxford English Dictionary:
Sacrifice • noun 1 the practice or an act of killing an animal or person or surrendering a possession as an offering to a deity. 2 an animal, person, or object offered in this way. 3 an act of giving up something one values for the sake of something that is of greater importance.
Or....being 95% of the way through a JD Robb book and lending said book to your daughter when she asks if you have anything interesting to read that she could borrow. (No, not me, but a colleague. She had one chapter, yes, one chapter to go....)
Personally, I think I would calmly explain that I had almost finished a fantastic book and that they could borrow it as soon as I was finished (and that I would try to finish it quickly for them). So, would you lend a book to a loved one even if you hadn't quite finished it?