The historian's role lies not so much in discovering and classifying sources as in interpreting and explaining them.
Literal
Historian [of-の] role [topic-は] material [of-の] discovery or classification [than-よりも], rather its interpretation [and-や] explanation [in-にある].
A formal academic argument. よりも ('more than,' the emphatic comparative) + むしろ ('rather, sooner') is a set phrase for 'not so much X as Y' / 'rather than X, it's Y.' や is the non-exhaustive 'and' — 'interpretation, explanation, etc.' (contrasted with exhaustive と). ~にある ('lies in') marks the locus of the role.