Full reading list in reading order for The Underworld Chronicles:

Inferi

Styx: The River of Hate

Oracle of Tartarus

Betrayal in Elysium

Guardian

Ferry of Woe

Monster

Aftermath

Redemption

Sinful

The Reaping

Lords of the Underworld

Retribution

The Hybrid

Cursed

Deliverance

Genesis

Dark Requiem

Fear the Reaper

Empty Pleasures

Firstborn

The Broken Melody

Reversal of Fortune

Revelations

Bloodline

The Dragon