We found him at his books in the library.

参考译文：我们发现他在图书馆用功读书/看书。

主语：we(我们）

谓语：found（发现）

宾语：him（他）

宾补：at his books（用功看书/读书）

状语：in the library（在图书馆）

at one's books用功读书/看书

He will stick at his books till midnight.

他要用功读书一直到深夜。

at 忙于，从事于;处于...中

I must go and see what those children are at.  

我得去看看那些孩子们在干什么。

They were at their meal.  

他们正在吃饭。