Here's your first tidbit:
There are plenty of picture books about the water cycle, but if you've been looking for a picture book about the carbon/oxygen or nitrogen cycle, you know those are harder to come by. Look no further than Why Do Elephants Need the Sun? by Robert E. Wells. Using an animal with terrific kid appeal, the African elephant, Wells explores not only the water cycle but the carbon/oxygen cycle as well. He doesn't explicitly address the nitrogen cycle but it would be easy to explain using the illustrations in the book.
Why Do Elephants Need the Sun? would be a great book to introduce during week eight of Classical Conversations cycle 2 when the parts of the sun are introduced because not only does Wells discuss the parts of the sun and how they work but also how the sun drives the water and carbon/oxygen cycles. Astronomy and ecology often remain very separate subjects for young children, all of those distant objects and lights out in space versus the bugs, leaves, and worms it is possible to touch, gather, and collect. Wells does a great job of showing how interconnected the two subjects really are. The text also touches on the planets (week 9) and states of matter (week 13) making it an all around great book to use with your classical kid for reinforcement and review.