Research is a form of inquiry, and students who succeed at research approach it with an open mind. Every semester, a handful of students come to the library reference desk asking for help finding sources to plug in to papers that they have already written. It may be helpful for you as faculty to join librarians in reminding students that research should start with a research question, and cherry-picking facts does not constitute effective or academically rigorous research.
Due to the small size of our print collection, we recommend that professors don't limit students to using print books in their assignments. We have an excellent collection of eBooks designed to support the research needs of our students in their assignments. The eBooks in our collection are from reputable academic publishers, and most if not all are available from the publisher in print editions with the exact same content. Due to our budget limitations, in many cases we may not be able to buy print copies of a title if we already own the eBook. With these considerations in mind, we hope that you will consider allowing the use of either eBooks or print books in research assignments where the use of books is a requirement.
In assignments requiring primary sources, it is helpful to be specific about what defines a primary source in the context of the assignment. Because the primary or secondary nature of a source is defined in relation to the event in question, a source may be considered primary in one context and secondary in another. (For an excellent overview of this topic, see "What Makes a Primary Source a Primary Source?" by Stephen Wesson at Teaching with the Library of Congress.) Due to these ambiguities, students are more likely to succeed in the assignment if you are as specific as possible in defining what type of source students are required to use for an assignments. In the sciences for example, rather than assigning students to use a primary source, consider using more specific terminology such as "empirical research" if that is what's required. That will also help us as librarians to assist your students in finding the resources required for the assignment.
For for more on the nature of primary, secondary, and tertiary sources, see this guide by the Libraries at Virginia Tech.