This special category of books includes things like encyclopedias, handbooks, guides, yearbooks, almanacs, etc. Reference books are most useful for obtaining background information on a subject, identifying key names or terms related to the topic, and finding (via a bibliography) the most important published works on a topic. There are reference books that cover general topics (like the Encyclopedia Britannica), but there are very specialized encyclopedias as well.
While a reference book might not be a source that you would use when writing a more extensive research paper, it could provide useful evidence or support for your recommendation proposal.
"Books" are probably what most people think of when they think of libraries. A book offers full-length treatment of a topic or subject. While academic libraries like the University Libraries do include some popular fiction and the literature that is studied by students and faculty in the Humanities, the bulk of their collections are scholarly, academic non-fiction titles.
You may not necessarily need or want to read an entire book, but you may identify chapters or pages that are relevant to your topic. (This is where the table of contents and the alphabetical index at the end of the book come in handy.) For some subjects, there may be a seminal book that is the basis of many discussions or examinations of a topic. If that is the case, it would be good to include and/or cite it.