To find materials that are not held at Bowdoin, search MaineCat. You can do this from within a failed search in the library catalog and then request materials using your name and campus ID. Still not finding the item you're looking for? Search WorldCat, and use "Check Availability @ Bowdoin" to fill out an ILL form. Another option is to search Google Books.
Subject searching generally provides a more organized view of the materials owned by the Library. Here are some examples of useful subject searches for Germanists.