OK, first, Audio and MIDI are two completely separate things. Audio is sound and MIDI is digital control signals for electronic instruments and computers (has no sound information at all).
To get audio into your iBook, you will need a USB or Firewire analog-digital/digital-analog audio interface. Some of these interfaces have MIDI interface built in, or you can buy a separate USB MIDI interface. Some keyboards have the USB-MIDI interface built into them too.
The Griffin iMic is the lowest cost (no MIDI). M-Audio and Edirol have a plethora of products from reasonable price up to pro level for audio and MIDI.
There are a "few" threads already on similar subjects (did you search?)
Details and links in here
https://forums.macrumors.com/threads/104122/
https://forums.macrumors.com/threads/121041/
https://forums.macrumors.com/threads/136629/
https://forums.macrumors.com/threads/139392/
https://forums.macrumors.com/threads/133344/
https://forums.macrumors.com/threads/136481/
https://forums.macrumors.com/threads/135904/
https://forums.macrumors.com/threads/134346/
To give any moree specific help we need to know much more about what you're planning to do.
Thanks
Trevor
CanadaRAM.com