I think I've found the theme for Book 4 of the Adam Cranford series. Whilst each of the books has action adventure there is an underlying plot for teenagers (and adults) to consider.. In Teen Valour I touched on population control. In Company Mole it was illegal drugs. The recently written Dangerous Donkey mentions the issue of green fuels.
For the fourth book I'm thinking about looking at the issue of global water shortages and what mankind can do to recover water in the planet. As part of my initial research I came across a video on Re-greening the desert. It is fascinating stuff. It shows with a little knowledge and some physical effort it is possible to fight back against desertification. Here's another link from the same source. My wife would kill to own this garden,