Musician's Guide to MIDI

The book, while a bit dated (chapters on older keyboards' quirks and such may not be very useful to musicians with new gear), does contain excellent descriptions of all types of MIDI messages, as well as hardware implementation details and flowcharts for implementing MIDI data reception, in case you're writing a MIDI program or designing a MIDI hardware gadget. The MIDI protocol has remained essentially unchanged for long enough that this book is still a great reference for MIDI musicians, software and hardware designers.

Rare is the book that, when faced with as complex and crufty a standard as MIDI, can explain it in terms that you can pick up the first time, and still be useful as a reference later. This is that kind of book -- useful for exeryone from the mildly curious, to serious MIDI developers.