"Reference sources are generally the place to begin your research, especially when you're starting out with an unfamiliar field. But they're also where you return when you need to look up formulas, facts, definitions, and other standard details; they tend to pack a lot of information into simple, easy-to-use packages." -- UCLA Library
Reference materials are generally not cited as they have not gone through the peer review process which means they have NOT been vetted by experts in the field before being published.
Be sure to also check out the materials your instructor has put on course reserves in Regenstein. Click on Library Reserves in Canvas to access the list.