I recently finished reading Learning CoreAudio by Chris Adamson and Kevin Avila, and for anyone looking at working with CoreAudio it is invaluable. Chris and Kevin take you through the complexities of the various CoreAudio APIs in a simple to understand way with a lot of very good examples. It is highly recommended.
It covers from the basics of AudioQueues right down to the lowlevel AudioUnit API, and also covers CoreMIDI and OpenAL on both OSX and iOS.
