Every book and spin-off of the original series, Dark Secrets.
Perfect reading order:
Dark Secrets
The Heart's Ashes
Mark of Betrayal
Lies in Blood
Echoes: Part One of Echoes & Silence
Silence: Part Two of Echoes & Silence
Bound 1
Bound 2
Bound by Secrets
In Another Life 1
In Another Life 2
In Another Life 3
In My Blood
Heart of Darkness
Underworld