Some reference books are sources , like an encyclopedia. They contain the information you need and are the original source for that information. Some reference books are collections of excerpts that are reprinted in one convenient place. You may find essays or reviews gathered from books, journals, interviews, or other sources. There is usually a citation back to the original source.
Some reference books are indexes to other books or journal articles.
They contain citations, maybe an abstract, that will lead you to another source such as a book or journal article. The next step is to locate the source in the library or request an interlibrary loan to borrow the item from another library. Online resources are available. Some are electronic versions of printed resources. Some are journal articles.