Don't confine your searches to just books or journals. 

Below are some other types of material for which to search.

Check dissertations and theses, both because they are part of a full-fledged literature review but also because they can contain useful literature reviews and bibliography. 

To search for dissertations and theses done at Lehigh or elswhere, see this library guide:

Preprints are a form of publication in which researchers post an article that has not yet been published in a scholarly journal, though it may have been submitted for peer review.

You may want to look at preprints as a way to track newly developing research and as one more source of bibliography, as well as a source of prospective bibliography, given that the preprint may see publication.

Some bibliographic databases now include coverage of preprints. Also, identify the major preprint server(s) for your field and search them for recent literature in your field.

"Grey literature is the unpublished, non-commercial, hard-to-find information that organizations such as professional associations, research institutes, think tanks, and government departments produce." From Grey Literature  (University of Michigan Library Library) 

See the list of grey literature types on the right side, as well as the link to a search guide at the bottom of the page.

If your research concerns a very applied area, consider looking at the literature view section of patents. From a USPTO glossary:

"NPL
Non Patent Literature -- documents and publications that are not patents or published patent applications but are cited as references for being relevant in a patent prosecution. For example, a magazine article or doctoral thesis relevant to a claimed invention might be cited as non-patent literature. Typically, references cited in an application are grouped into: domestic patents and patent application publications; foreign patents; and non-patent literature."

See also the library guide about patents below:

If you are doing humanities or historical work, contact Special Collections for primary historical books, manuscripts, and letters.