Kat Dubois Chronicles

The Kat Dubois Chronicles is the second series in the Echo world, which includes:

Echo Trilogy

Kat Dubois Chronicles (and the Nik Chronicles serial on Ream)

Fateless Trilogy

The Echo Trilogy Reading Order:

Book 1: Ink Witch

Book 2: Outcast 

Book 3: Underground

Book 4: Soul Eater

Book 5: Judgement

Book 6: Afterlife