Not sure this one qualifies as being part of the "main repetoire", but Max
Reger's first violin sonata, written when he was 17, was an opus #1, and in
my opinion it easily makes the ranks of enduring music.  I'm still trying
to make up my mind about Reger- one of his best known works, his clarinet
quintet, leaves me cold, for example- but this youthful violin sonata is
a delight that helps keep me listening to the composer.

A few weeks ago, someone claimed Mozart lacked emotion.  The thought
crossed my mind that perhaps she had momentarily confused Mozart and
Reger, but when I listen to some of his violin sonatas, I say, "Nah!".

Larry