Try messing around with buss processing and track sequencing while referencing other recordings in the same genre. Just make sure to keep the unprocessed tracks around, and err on the side of "less is more". There's a lot of overlap between the aesthetic skills needed for mixing and those needed for mastering.

The profession of mastering is much more than that, but generally when people ask this question they want to know about the aesthetics, not about stuff like thoroughly grokking many source and delivery formats, etc.

I don't think there are. Lots of resources on technicalities of the craft, but I'm not sure you can effectively teach mastering skills without hands-on approach. Some subtle nuances of the sound are just very hard to describe and be sure the reader really hears it properly.

Mastering engineer needs to develop sharp focus, integrate themselves deeply into their listening environment and setup to understand how their decisions will translate in the wild. It's also important to know what the author/producer and the listener expects from the music, so you can, fortunately, make both sides happier. This implies listening a lot of different music with a critical ear.

I mostly do mixing, but do mastering as well and I enjoy the process a lot. It's not easy but can be extremely rewarding.

The book Mastering Audio by Bob Katz covers a lot of ground.