Georg Philipp Telemann

Telemann wrote more music than any other composer in history — well over three thousand works — and he wrote a great deal of it deliberately for competent amateurs, which makes him unusually useful to anyone who is not a professional.

The recorder and flute sonatas, the Fantasias for solo flute and for violin, and the trio sonatas are the core of it, and they sit at a difficulty where a good amateur can play them properly rather than approximately. The Tafelmusik collections are the same music at chamber scale.

Almost everything is out of copyright and freely available, but continuo realisations vary in quality enormously and a bad one makes the music sound stiff. For anything you intend to perform, the realisation is what you are paying a modern edition for.