Why do muslims refer to christians and jews as "people of the book"?

Muslims believe that once before Islam was a religion, their were other religions, but all fell under the category; Tawheed. Tawheed was believing one god, which was in what they believed, Allah (swt). Since all of the following religions have a specific book that used to fall under Tawheed, they are the people of the book. 

3is faulse becaues the big so that wpuld mean thtmat they are farther apart.

The congress completes the diagram