The case for Pluto
Neptune had been discovered based on its gravitational effects, and it seemed that there had to be another planet out there, causing similar effects. In 1930 Pluto was discovered, but it was soon realised that it was nowhere near as big as it should be - should it still count as a planet. The discovery of its moon Charon enhanced Pluto's status for a while, but it was gradually realised that there were likely to be many other objects similar to Pluto, and indeed in 2003 an object larger than Pluto was found. Then there's the asteroids - it's hard to find a definition of a planet which includes Pluto but excludes Ceres. And what about Exoplanets, do they all deserve the 'Planet' tag? Boyle tells of the IAU meeting which was supposed to resolve the status of Pluto, but how arguments continued for quite a while afterwards.
It's a well written book, easy to read, and whilst not a children's book, it's accessible to a high school student. I think it would make an excellent gift to anyone you know who's keen on astronomy.