here could be links to Reference Books, eBooks, online/subscription encyclopedias we may want to break this down
A "Reference" source is media you "refer" to for either basic/background information or a specific piece of information. They differ from other sources in that you rarely use them "from cover to cover". Reference sources are also usually organized in a specific way (alphabetically, chronologically, etc.) to make finding information quicker and easier.
NOTE: it is very rare to cite a reference source as evidence in an academic work. Reference sources can be physical or digital (on the web or in a database.)