What do books have that journal articles generally do not?
At the beginning of the research process, when you're not very knowledgeable about your topic or the issues surrounding it, books will give you background information, or context.
They introduce vocabulary related to your topic--words that will help you search for more specific resources on your topic later.
They cover more of the history of your topic, because books are made to last. The same book will continue to inform researchers through time.
What do journal articles have that books generally do not?
When you have a basic understanding of your topic, journal articles will give you information on more specific issues within your topic, often relative to a certain quantitative or qualitative study.
They contain a lot of statistics--data that you might use to support or negate a claim about your topic.
They cover current issues from the time that they are published, because they are produced quickly. A journal article will keep a researcher up to date on the newest information.