Duke's Rare Book, Manuscript and Special Collections Library has excellent resources in architecture, including folio volumes of plans. Because of the nature of its collections, its riches are not as discernable through the Duke catalog as other material. In addition to doing a search limited to Rubenstein Library, email or query a special collections librarian for a consult on what may be of interest. RUBENSTEIN SEARCH
For example, type in ARCHITECTURE GERMANY in subject field.
WHY BOTHER DOING ARCHITECTURE RESEARCH RIGHT? (No, seriously) Four quick videos
Google Books and Google Scholar (journal articles) provide key-word searching for billions of books and journals. This is a huge help to researchers because Duke indexes, such as the Duke Catalog and many journal indexes do NOT search the texts word, only the catalog record. Of course, you can only see a snippet of where the words you're looking for lie. Just copy the title of the book or journal (journal title, not article title) and enter it into the Duke catalog.
Google Books - books.google.com
Google Scholar - scholar.google.com