This is a hugely enjoyable novel very much in the territory of Douglas Adams, or Red Dwarf. A mathematician working on the Riemann Hypothesis disappears. When he reappears, he behaves oddly, and has amnesia. It transpires that his body has been occupied by the consciousness of an alien, sent here to slow humans mathematical progress, as we are a too dangerous race to be allowed to discover interplanetary travel. It's an old premise, much used in serious and satirical fiction, but Haig pulls it off with originality and wit. His observations of human failings, and his affection for our strengths - love, especially - make the book surprisingly moving in parts, as well as very, very funny. Not heavy, but not trivial.