This entire series steals a lot from different genres. First of all there's the detective, John Taylor, who reminds me a lot of John Constantine of the Hellblazer comic series. They both share the same name, have mysterious backgrounds and spend their entire lives trying to find out who (or what) they are. Coincidence? I think not. Secondly the background seems to me part Terry Pratchett, part Anne Rice and part Monty Python. There are vampires and angels and demons but they all have their own hidden agendas. It's not just about Good and Evil there's also the Establishment to consider.
This entire series is just one crazed supernatural whodunit fest. While that's unique in itself what I like about this series is Simon R. Green's unique plot lines (carnivorous cars? singing till you die?) and undercurrent frisson of horror. You never quite know who'll be the next to go in the Nightside.