libmidi is a very simple library created for reading and writing MIDI files. It is designed to be quick to use and quick to learn.
All interaction should be done through the MIDI::File class, which parses a MIDI file. The data inside the MIDI::File class is then organized as if it were a normal MIDI file.
That is, MIDI::File has a header (MIDI::Header) and a list of tracks (MIDI::Track).
Each track then has a list of MIDI events.
See the MIDI file specification for more information. http://www.midi.org/techspecs/index.php
The following sites also describe the MIDI file, but are easier to read
1.6.1
