Some of them ask for proper references, i.e., scientific publications about a given topic, while others ask for more general resources such as books on broader topics.
Considering the low number of questions, I don't think we should split them across two tags, but then the "reference" of the tag label could be misleading and prevent it from being used. I suggest that we do something like Astronomy SE: they have a resource request tag, with a wider definition:
Questions asking "What is a good book about X?" or more generally, "What should I read to learn about X?", typically where there is not a single authoritative reference. Also requests for multimedia, like video lectures.
and reference-request is just a synonym of this tag. Any thoughts?