Old Blood is a novella from the POV of Pippa (or the Crazy Clown Lady) who in the last book On Demon Wings we found out was once Dex's nanny ****spoiler begins****and Perry's grandmother - who was thrown into an asylum by her mother and father and eventually died there - alone and forgotten****spoiler ends****.
Pippa's tale is a sad one. She too shared Perry and Dex's ability to see ghosts and this 'curse' is what eventually led to her being committed. It begins with Pippa as a small child becoming aware that she can 'see' things no one else can. Years later, after running away to Stockholm become an actress, she ends becoming involved with a handsome but selfish young actor called Ludwig. This relationship will culminate in the birth of a daughter but will eventually become her undoing. Pippa too is visited by strange apparitions ****spoiler begins****including the spirit of a ginger-haired teenage boy calling himself Jakob (could this in fact be Maximus Jacobs or am I clutching at straws here - I just feel there is much more to that ginger douchecanoe than meets the eye??****spoiler ends****.
In Old Blood we see Dex as a young child and this insight into what his young life was like was both fascinating and troubling. His family...is disturbing...to say the least and Pippa is the only stability this young boy has.
Not necessarily essential reading (you could skip it and not miss out on too much) but it definitely enriches the backstory behind the series and I would recommend fans of the series to pick this up.