Journal articles are useful because they often contain the most up-to-date research analyses and discoveries. Many journals are also "peer reviewed": this means that every article is evaluated by an anonymous panel of scholars before publication. More information about Peer Review can be found here.
Journal articles are shorter than books, making them a little easier to digest for your literature review; however, keep in mind that journals are intended for a scholarly audience of professors, professional scholars, and graduate students. While journal articles may be a more challenging read for the novice or younger scholar, they are an extremely important secondary source for literary research. Please don't hesitate to ask for help!