问个简单的英语语法问题.

问个简单的英语语法问题.
He is reading a book.
我知道he是主语.is reading是谓语.
宾语是a book 还是book.a做什么成分?
还有一句
We were talking about you.
we是主语。were talking是谓语。
about you是什么成分？