Recently, I have been pushed toward studying analytic continuation dirichlet series with twists that are additive. These are functions $D(s) = \sum \chi(n) \frac{a_n}{n^s}$ where $a_n$ is some sequence (usually positive) and $\chi : \mathbb{Z}_{k} \to \mathbb{C}$ such that $\chi(a+b)=\chi(a)\chi(b).$
I would like to know if there is any literature on this topic available.