Once I asked my friend Alex if he had read a particular book, Deborah Tannen's Talking From 9 to 5. He quickly replied, "yes, sure!" A few beats later, he said, "Actually, I read about half of it."
He said he has read a book, even when he has only read part of it. So, for him, the part stands for the whole. I regret that I don't have enough confidence in my reading abilities to be able to say that I have read a book when really, I have only read a subset of it. What if I miss that seminally important point of the book? What if the narrative takes a bizarre twist right after I put it down? It's like walking out of a party before it reaches full-blast. You won't know who arrived, you won't know when people left, who met whom, in what combinations people socialized, etc.