Volume 2 contains the basic physical ideas and laws used in the study of the outer layers of a star including stellar magnitudes, spectra and temperatures, radiative transfer in a stellar atmosphere, line formation, the spectrum of hydrogen, spectral analysis, and available structural components.
I've read all three of these a number of times, but I use Vol. 2 the most for lecture (In a 10 week quarter it is hard to get much beyond atmospheres). I've never found anything better than this for undergraduate senior level stellar atmospheres. I'm currently teaching out of this book and the math is perfect: I can give about 2/3 the steps in class, but anyone left behind can follow along from the text. The clarity of the conceptual explanations of the physics is on the level with Giffiths E&M. Other texts either give too many details (grad level) or too few (most current undergrad level - honestly how am I supposed to convincingly explain the H- contribution to stellar opacity without doing the full radiative transfer derivation? Seniors can do this math, they've had Stat Mech).