With his attention ___ on the book he was reading ,he didn't notice what had happened.

with复合结构的用法。with+N.+Ving/Ved/to do
名词与动词是主动时Ving.
与动词是被动时Ved.
表示将来的动作时to do
focus one attention on sth.
attention 与focus 是被动关系。

focus 是用来修饰 attention的，因为如果是人focus，那就用原型，可是这里是attention 被focus，所以用被动语态。