Here’s a recent talk from the game designer for the original Sonic games. It’s mostly about game design in general, but he talks a little bit about designing Sonic 1 and 2 near the beginning.
He shows some never-before-seen concept art, including the original Green Hill level map (drawn on draft paper!) and scrapped concepts for a world map for Sonic 2.
He’s also been sharing more concept art on Twitter. Also be sure to check out Naoto Ohshima, the original character designer of Sonic and friends. He’s been uploading his own concept sketches, including for NiGHTS and Blinx the Time Sweeper.