I've read books like Theory of Harmony, Tonal Harmony, The Complete Musician, and just started reading structural functions of harmony. But I still find myself confused on those weird out-of-key chords you find in songs, like chords that aren't found in common practice. What could help me in analysing chords better?
But I still find myself confused on those weird out-of-key chords...
I'm not sure how helpful it is to think of out-of-key chords as "weird".
A key doesn't tell you "You must use only the notes I say, and if you don't, you are weird".
A key says "If you build triads within this set of notes, that's a good starting point for creating some harmonious sounds". But it doesn't imply that anything bad is necessarily going to happen if you move outside the key. In fact in minor key harmony it's incredibly common to step outside of the notes that are diatonic to the key - hence the existence of different versions of the minor scale.
Of course if you are thinking in terms of keys, then perhaps you need to have some ways to think about motions that take you out of a single key. Some ways to think about these are:
- as pivot chords, or secondary dominants, leading to modulations or new tonicisations
- as borrowed chords
- in terms of a harmonic 'palette' - i.e. the use of the same type of chord based on different degrees on the scale, and seeing chords as being used for their 'colour' rather than function
- in terms of voice-leading and melodic consonance/dissonance
Ultimately, thinking in terms of keys is only one of the possible ways of thinking about harmony. They are a way that has proved to be quite a good match for quite a lot of Western music, but they're a cultural construct, not something fundamental to the order of the musical universe. If Western musical culture had evolved differently, it's quite possible that we wouldn't think in terms of keys in the same way.
It's also still possible to be weird while staying within a key too! The diminished vii chord in a major key is a very dissonant, clangorous, ugly duckling of a chord. If that sound is 'allowed', you shouldn't worry about having license to do anything else in music!