How to search across different documents?

I often open more than one books and need to search key words across these books, but only one book is included in search results. I don’t know how to solve the problem.

I think this feature can only be achieved if you add those document to one notebook, but for index global search this feature is not available in MN3 …

I want to clarify my question here. I used Manage button to import book B to book A which has been opened, that means now I already have 2 books in this notebook ,right? Then I searched with the word of my interest in Documents, but the searching results are all from book A. It seems the search engine didn’t search the word in Book B. Another possible explanation is there is some place I could set in which book(s) I want to search the word, but I never see it. Thanks for your further reply.