ANN Link
Seeing as though it's not stated, i'm gonna guess that it will be it's own story based on the novels, and not a sequel to the TV series. I'm sad it won't be, though at the same time glad we're seeing more from the series. I absolutely loved the anime, would love to see more.
Interesting that the ANN artical mentions Marina Inoue regarding her tweet. I hope that means she'll be voicing Kasahara again.