Almost all the material out there on mixing explains tools: how a compressor works, what each knob on an EQ does. Very little explains the decision that comes first — the one separating people who progress from people going in circles for three years: faced with a mix that sounds wrong, what exactly is happening and what should you touch?
That skill has a name in every technical trade — diagnosis — and it's trained the same way as any other: seeing many cases and checking your reasoning against the right answer. A mechanic doesn't learn to fix cars by reading the manual for a spanner.
Each scenario describes a concrete situation — the vocal isn't intelligible, the mix falls apart in the car, kick and bass are fighting — and offers four possible interventions. There are 12 scenarios. The useful part isn't getting it right, it's reading the explanation when you don't: the wrong options are written to be plausible, because expensive mistakes in a real mix are never absurd. They're the correct solution to a problem you didn't have.
Diagnosis has two requirements and this covers one; the other is actually hearing it, which is what ear training is for. Two cases that come up constantly have their own guides: mixing vocals and kick and bass.