> I'm most fascinated by spectroscopy,

Ha! As an astronomer and primarily a spectroscopist, this warms my heart, but it is definitely a case of "one of these things is not like the others".

Without going too technical on the subject, the main things I think you should learn about spectroscopy are:

• Some basics on atomic physics, why we see discrete spectral lines from electronic transitions from different ions and molecules.
• The difference between spectral lines and continuous emission processes, such as blackbody radiation, bremsstrahlung, etc.
• Some basic radiative processes: where do we see these processes, what they look like, and what we can learn from an object based on these properties?
• How does motion affect these features (redshift and blueshift)
• Some basics of how spectrometers work.

I know its usually a bummer when someone recommends a textbook, but slightly more detailed spectroscopy is not really popular sci-comm. If I remember correctly, 21st century astronomy is a good entry-level textbook.