Miller Library has access to a wonderful database called Mometrix that offers PDF versions of every test prep book they offer in print. 

You can also access some practice material using the website at Michigan eLibrary or http://www.mel.org/. Once on the MeL website, select Practice Tests from the left sidebar. Choose College Students and type the word "CLEP" or "GRE" in the search box at the top of the page.