If you have a math/physics/chem background this is a good book [0]. I used it in my grad course on spectroscopy, it does a decent job walking through the concepts step by step. Spectroscopy is fascinating though, I used Raman Spectroscopy for my research, the amount of information you can get out of some lasers, mirrors, and gratings is amazing.

[0] https://www.wiley.com/en-us/Modern+Spectroscopy%2C+4th+Editi...