The core part of the sentence is
There is a common European myth to which both authors referred.
You could split this as
There is a common European myth. Both authors referred to it.
Where "it" means "this common European myth.
The rest of the sentence tells us how and where:
There is a common European myth. Both authors referred to it [subconsciously] [in the books in question].
The preposition "to" has been moved with the relative pronoun to observe the rule "the pronoun goes before the noun". If you choose not to follow this rule you get the equivalent sentence
There is a common European myth, which both authors referred to subconsciously in the books in question.
Which is correct, has the same meaning, but is slightly less formal in style.