The following magazines often publish articles that relate to MIDI, as well as related subjects such as synthesizers, etc.
Electronic Musician (6400 Hollis #12, Emeryville, CA 94608)
IMA Bulletin (5316 W. 57th Street, Los Angeles, CA 90056)
Keyboard Magazine (2800 Campus Drive, San Mateo, CA 94403)
Roland User's Group Magazine (c/o Roland Corp. US, 7200 Dominion Circle, Los Angeles, CA 90040)