5 sub-series, each 3-4 books. They are pretty short, so they go fast.
Series 3 is a sequel to 1, series 4 is a less-direct sequel to 2. Series 5 acts as an ending for 1/3 and epilogue for 2/4. I believe she is currently writing series 6, which will follow one of the main characters from 5. It's all written chronologically though.