Many thanks to SWLing Post contributor, Eric (WD8RIF), who writes:

This came in the most recent ARRL Contest newsletter:

The Sun, The Earth, and Near-Earth Space: A Guide to the Sun-Earth System by J. A. Eddy is a readable and accessible textbook that explains the dynamics of the Sun and its interaction with the Earth’s ionosphere. It’s available as a free download, courtesy of NASA and the International Living with a Star Program. Anyone using the ionosphere as a medium for radio wave transmission and wants to better understand propagation should find this book of interest. (Ward, N0AX)

Thank you for the stellar tip, Eric!