It provides good general information and cover many aspects of MIDI mesages and MIDI instruments. It is definitely useful for people who knows liitle about MIDI. For people who knows what a digital music sequencer (hardware or software) produces and already known and manipulate (add or edit) some MIDI CC (controle codes) to change timbres, alter audio effects, it has liitle value.
It still provides good BASIC level knowledges about MIDI and modern audio manipulations to novice users.
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The MIDI Manual