The most obvious are books and journals, but there are some loose rules about their uses. Books are very helpful for finding historical material. However, reviewers should always ask themselves if the material in the book is outdated. For example, to discuss the history of forest preservation, one might want to use both older and newer sources in order to contrast and discuss the perspectives of people in the 1920's versus people later, but to discuss procedures for carbon dating, newer sources must be used in order to be sure of having up to date information on the latest scientific advances. A danger in using books too heavily is that books often contain the considered opinion of researchers but not necessarily hard data to back up considered opinions.
Books are, however, a good source of theories.
Journals are particularly helpful for many projects because articles found in them normally pass through a review process and are targeted for an academic audience that is looking for refereed information. Journals are an excellent source for studies pertaining to IQP topics. It is in them that students will find the variety of perspectives and conclusions based on analysis of data. As mentioned before, they should not discount views they don't like. Such views are essential to putting the story together.