Well, you'll have to do it in real time, which isn't quite as fun as ripping a CD.
However you could encode the recording of audio data at the same time.
I just downloaded this and haven't tried it yet. Audio Recorder, it is supposed to do AIFF and MP3 on the fly.
Presumably you have got the cables to connect cassete player to iBook sorted?
I use Bias Peak to record. They have a free trial. Your real challenge on an iBook is lack of a line-in. You'll need something like Griffin's iMic, which plugs into the USB port. I've had noise problems with the iMic though.
Although it is ugly, it does the job of recording quite well. I've even used the noise reduction filters to good effect on some old LP's I was converting a while ago.
I use Bias Peak to record. They have a free trial. Your real challenge on an iBook is lack of a line-in. You'll need something like Griffin's iMic, which plugs into the USB port. I've had noise problems with the iMic though.