If you want to learn about the life of a particular person, or the history/background of an issue or topic, a book can cover much more material than an article. Some books collect important essays discussing many angles of a topic, and many books are written by scholars; however, books take time to research and write, so while books have in-depth coverage, they might not always have the most recent information. Books are a great source to get an overview of a topic.
Sometimes periodical articles are sufficient for a research paper, but often you will need to combine books with magazine and journal articles. If your topic is not too recent or specialized, books might be a good option. The large amount of periodicals published makes it more likely that you will find articles on very narrow subjects which might not be easy to find represented in books.