There’s lots of little gems of advice in this piece from the Guardian where 5 songwriters pick their own favourite songwriter.
In each case there’s a bit about what they think makes the other great – technique, subject matter, etc.
For example, Dianne Warren on Paul McCartney:
As a songwriter you need a gift for melody, and for saying something you’ve heard a million times in a different way. There are only so many notes and ideas, but you twist it and turn it and make it new, by the chords or melodies or a weird key change. In just a few years, the Beatles created this genius body of work, and those songs will never go away.