Basic Midi is a great starting point for anyone who has heard of MIDI and yet feels completely confounded as to what it is, what it does and how to make best use of it. It has plenty of background and useful explanations. However, it is good for beginners. If you’re an experienced MIDI user then this isn’t going to be nearly as much use to you. I’ll leave it up to you to decide.
You can find it here on Apple iBooks.