Selecting the right database can be the best way to find relevant sources. You may want to start with a general database, like Academic Search Ultimate and then move on to a more specialized one like Gale Literature. If you can't seem to find anything on your research topic, try different keywords or another database. Remember, you can always ask for help by using Ask A Librarian.
When your professor requires that you read and cite only peer-reviewed journal articles, you should exclude any book reviews. Book reviews may be acceptable sources if the author or work is very contemporary and little or no literary criticism has been published. Furthermore, most book reviews include a great deal of analysis and interpretation. However, reviews lack footnotes or citations and are not peer reviewed. Overall, a book review is the opinion of one professional book reader.
Not all book reviews appear upon the first publication date of a book. A film version of a book may appear (for example, Bambi, by Felix Salten--later a Disney movie), and this event may lead to brand new reviews of the Bambi novel in cultural magazines like The New Yorker and Harper's.
Book reviews may be exactly what you are looking for. For example, you may want to read and cite book reviews if your paper is a study of the reception of a book in popular media over the decades.
One clarification on the chart: It's not exactly accurate to say that book reviewers have no formal training in literary criticism. That is an exagerration to make the point that literary criticism almost always appears in scholarly journals, where professors publish their new ideas. Also, literary criticism appears in books, not just journals. For example, all the books edited by Harold Bloom are works of literary criticism.