While SearchPlus will return both books and articles, you will find the library catalog to be a more focused tool for finding scholarly books, both in paper format and e-books. Your instructor is limiting books to those published by a University Press as a convenient restriction to ensure the scholarly nature of any books used.
To find these, search by keyword and include 'university press' in your search. Figure out the jargon & vocabulary used by whatever search tool you are using. A word that works well in one search tool might not work well in another search tool. The sample searches below go from sloppy to smart in the library catalog:
Try your search again without the 'university press' part to locate other books and government documents on your subject!