这句话里:“book in hand”做啥成分啊?

问题描述:

这句话里:“book in hand”做啥成分啊?
Our teacher entered the classroom,book in hand.
这里的“book in hand”做什么成分啊?
为什么book前面不用冠词啊?

Teacher handed a book to enter our classroom.
这时候才要用冠词,贵题的book in hand是做伴随状语.