I just discovered this site, a collection of expositions of the fugues in Bach’s Well-Tempered Clavier. Some of Tim Smith’s writings are pretty opaque to those of us who aren’t trained in music, but many of his comments are accessible enough. (“If you think of the subject as a dancer, then the fugal process is one of finding a suitable partner. But what if the dancer has the ability to be its own partner? Well that is stretto. And stretto is what the C Major fugue is all about.”)
And the visualizations help, although I wish they were done in Flash instead of Shockwave. But hey, it was made in 2002.