Prosper's Demon is a novella by K.J. Parker, a pseudonym of Tom Holt. It is the tale of a mortal exorcist fighting possessions by immortal demons, in a quasi-medieval setting. The exorcist discovers that the famous self-proclaimed scientific genius, Prosper of Schanz, is possessed by a demon, and the exorcist begins a forbidden collaboration with the demon, seeming to aid the demon's long-term plans because they will outlast the exorcist. The dual between the two is clever, but the story is only moderately successful, and the ending is rather rushed. Prosper's Demon is darker in ways than the other books by Tom Holt that I've read.