John Reginald "Jack" Spratt is the protagonist of the Nursery Crime series, a duology of fantasy alternate history sci-fi mystery novels written by Jasper Fforde. He is the current detective in charge of the Nursery Crime Division (NCD) and has been in the police force for 26 years; although he is often overlooked and underestimated due to working in the NCD, not being Guild-affiliated, and having a terrible conviction/investigation ratio because of the other two reasons.
Jack is also what's known in the series as a Person of Dubious Reality. He's an interesting case as he is based on two fairy tales and a rhyme.
Spratt and Mary were originally introduced into the alternate Fforde universe in the Thursday Next novel The Well of Lost Plots (2003) before taking over their own "Nursery Crimes" series: