A book in a series that explains our cultural holidays, that by necessity here turns into an eco-lesson. There is no purpose to having an Earth Day if it's not for the environmental reasons, and in describing the relevant activities, thinking and, if you like, celebrations, it has to inform us about the issues and what easy steps the bulk of us should be following to sort them out. (I don't know about the paragraph about space junk, mind – that might not be the greatest bit here for us to follow.) It's all fine stuff, but it does seem a touch out of place in the series as it stood up to now.