Why is the indefinite article used here? A novel called “Norwegian Forest”

The simple explanation here is that you're being introduced to a novel. The sentence later elaborates to tell you the name of the novel, but the indefinite article can be used simply because novel is an indefinite noun.

This construction, however, is likely simply idiosyncratic. It should be noted that both the definite and indefinite articles are grammatically correct in this specific use case (i.e., when the article introduces an indefinite noun which is later in the same sentence specified to be a definite, singular example of that noun). Which is used is up to the author; it makes no difference in how the sentence should or would be interpreted by a reader.