“MIDI“, an explanatory essay by Christopher Dobrian.
A short essay by Tom Igoe on MIDI provides some technical information concisely and clearly.
A pretty thorough-yet-readable explanation of MIDI messages can be found in The MIDI Companion by Jeffrey Rona, much of which is available online on Google Books. For understanding common types of channel messages such as note-on, take a look at the chapter titled “Channel Voice Messages“.
Website of the Music Manufacturers Association (MMA).
A summary of all MIDI messages provided by the MMA.
“Binary number representation“, a brief introduction and pointer to the web page “Representation of Numbers” by Erik Cheever. (Read at least the first two sections.)