Scholarly journals are full of articles that reflect the most recent experiments and research on various topics. Books, on the other hand, may be current at the point in time that they are written, but begin to lose their currency immediately, before they are even published. This is not to say that books are not good sources of information, but if you need the most current information on a topic, periodical articles are usually a better bet. Websites can also be highly current, in some cases more current even than newspapers. Websites can also be highly current, in some cases more current even than newspapers.
Books can offer an in-depth, even comprehensive view of a subject.
Articles, on the other hand, will be more focused on a specific aspect of a subject. For example, suppose you are writing a paper on a poem by Robert Frost. You can easily find a book on the poetry of Robert Frost in the library's collection, but that book might or might not discuss your poem in detail. However, you could probably find several articles that focus specifically on the poem you are interested in. Articles are great sources for finding information on specific facets of a topic.