I second Robert's list as a good way to get into stellar evolution & its physical tracers. I tote along the K.S. de Boer and W. Seggewiss and the Salaris and Cassisi books with me on every dark site visit. They are pretty well thumbed by now as quickie fact-finders whenever I need to refresh on some property or other. For some reason Dina Pralnik's book doesn't measure up in this league.
There is a freebie alternative to the Grey & Corbally "Stellar Spectral Classification" book available
here. Scroll down to "Download English Documents" & have a good read from his half-dozen self-produced books. These aren't dilletante productions by any means. There's a surprise in store on p.207 of his "Spectroscopic Atlas for Amateur Astronomers". A number of the high-res spectra he analyses were acquired with his own Celestron C8 kitted out with one very comely focal plane spectrometer. The massive compilation of internet links starting on p.208 of that same monograph is a freebie cornucopia of anything you could ever want in the field of astronomical spectroscopy. He even has one spectrum of a lightning bolt!
Thanks for starting this post, Robert. I hope you follow up with other recommendations in the many specialties that interest us.
=Dana in S Africa