Why should you use books?
Isn't everything online these days? The answer is NO. Most scholarly content is not found on the general internet.
There are good reasons to use books in your research:
Books can give you a detailed anaylsis of a topic.
Books will provide background information on a topic.
Books can give you overview of big issue
And you can use the bibliographies in books to find other sources.
Most professors like students to use a variety of sources, including books.
If you need to do some serious research, books are often a great choice!