My familiarity with Elvis Costello would not go too much past the more popular stuff, for me an offshoot of a period fascination with Ska. He produced an album or two for The Specials. I’d spent a little time in Jamaica at that point and had developed a taste for dub, somewhat related township music, etc. and heard echoes of it in his music. Never really pursued it too deeply but maybe I should.
I love the Ska sound without a doubt. It started for me with the English Beat and then reemerged with bands like Sublime and 311 as Ska went Punk.
As for Costello, if you can find almost any solo acoustic performance of Elvis, you will find gold. Those done back in the day shortly after their release. Extremely rare finds. In 2000 I was an active downloader from Napster and picked up a bunch of solo acoustic tunes that I have since lost. Youtube is worthless when it comes to rediscovering those tunes.
And to shift gears a bit, you should check out Regina Spektor.