According to the webpage, Charles got the idea of writing the Ladybird book following a discussion with his loyal MP friend Nicholas "Fatty" Soames after returning from the COP21 climate Change conference held in Paris in Dec 2015. He then approached Tony Juniper and the climate scientist Emily Shuckberg to write the book.
If this was true, you wouldn't expect the Ladybird book publisher (which I think is Penguin Books) to call this new series of books the 'Expert Series', as Charles cannot really be regarded as an expert on the science of climate change. He is more like an authority figure, and a term like say the 'Authoritative Series' would have been more apt. Also consider the other two titles in the launch of the 'Expert Series' - a book on 'Quantum Mechanics' by Jim Al-Khalili (Professor of Theoretical Physics at the University of Surrey), and a book on 'Evolution' by Steve Jones (Professor of Genetics at University College London). You wouldn't pick Prince Charles as being the nearest equivalent of those two expert authors for a book on climate change. That leads me to think that the originally intended expert author selected by the publisher was Emily Shuckberg, with Prince Charles and Tony Juniper then coming in at a later date. I can imagine Charles requesting (and not being turned down) that he be included as a co-author of the short book, along with his friend and adviser Tony Juniper, following what to him would be the shock result of Donald Trump being elected as US President in early Nov 2016.
Writing this sort of book suggests to me that Charles now thinks that he might actually have to convince the 'plebs' about climate change, which is not something I've previously noticed him doing. He has given quite a few speeches on climate change over the years, but he tends to give them to various sympathetic elite audiences like the EU, the UN COP conferences, and sustainable development people from big business. Until Donald Trump got elected, it would probably never have occurred to Charles that the opinion of the hoi polloi in the West towards Greenery mattered at all.