Q: Which dc term would you use to describe book series?

A: This is an example of where traditional DC is too generic. DCMI Terms (which superseded DC Qualified) is a little more granular.

http://dublincore.org/documents/dcmi-terms/

In tradition DC, the only option really seems to be Relation and use IsReferencedBy or by IsPartOf.
IsReferencedBy is for more of a citation type relationship, whereas IsPartOf encompasses a variety of relationships.

Here is an example using IsPartOf:

Also, LC no longer traces (provides an authorized access point) for series, but most large academic libraries (and others) still consider that to be valuable.

http://dublincore.org/documents/usageguide/qualifiers.shtml