By far and away the best guide to the professional use of MIDI is an article by Hinton Instruments entitled Using MIDI in a professional environment. This is essential reading for anyone considering using MIDI in any shape or form. (There's also some great history of the EMS synthesiser company, what a nice contrast of equipment of the times)
Another good reference on the same site is MIDI Protocol guide, which not only has in tabular form the MIDI spec manual, but also has a cross reference of MIDI Manufacturers' ID Numbers, which in many cases has a link to the manufacturers web site. About the only thing not on this reference is " how do I get a manufacturers code for system exclusive messages ", and the answer to that is you join the MIDI Manufacturers Association, and yes, this will cost you money!
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