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The MIDI Standard
The MIDI standard is maintained by the MIDI Manufacturers Association (MMA), who have an online presence at MIDI.org. The MIDI specification itself is a printed document, available from the MMA. This is a substantial document, which provides in-depth descriptions of many aspects of the protocol.
If you're casually experiementing with MIDI, there are plenty of reference materials available online. However, if you're serious about making MIDI instruments, this document is worth having. Joining the MMA as a manufacturer also grants you a System Exclusive ID code.