Reference materials are usually encyclopedias, dictionaries, handbooks, atlases, etc. They are intended to provide quick, reliable information, summaries of complex matters, and, very importantly, bibliographies--suggestions for futher reading.
These reference materials are invariably written by experts in their fields, so they are more trustworthy than Wikipedia--all have gone through a process of editorial review. For an illustrated explanation, see the home page presentation, "What is a referred or scholarly article?"
Note: I may use the term books here, but many reference sources are available online, full-text.