The novel serves as a travelogue through Pratchett's future universe, and with its cast of well-developed aliens and locations this is a rich journey. Probability maths, quantum physics and evolution are dealt with in a light and readable manner with a small but well drawn cast. The novel isn't laugh out loud funny, but there are elements of Pratchett's trademark humour.
A short but successful piece of sf universe building, recommended even for those who may not enjoy the authors comedic fantasy output. After re-reading this the real mystery is not the whereabouts of the Jokers, but why with his prolific and relentless Discworld output Terry Pratchett has never found either the time or the inclination to produce another science fiction novel - on the evidence of this he certainly has the talent.