We're working towards a cover design for the book. It's something a little different from many computer book covers. I proposed that we use a graphic design based on a koru, a traditional New Zealand pattern that is an integral part of Māori art. I wanted an iconic image that could both be tied to my homeland and also could also (loosely) connect to the subject of the book (the spiral pattern relates to the notion of moving inward from user space towards the kernel, as in when we make a system call).
Bill suggested that we instead use an actual photograph of the natural object from which the koru pattern derives. After some searching, I was happy to find a strong graphic image of a koru fern--specifically, a Black Tree Fern, or Mamaku (Cyathea Medullaris):
This wonderful photograph is by a fellow New Zealander, the well-respected nature photographer Rob Suisted (blog). Rob has several books to his name already, and his images have appeared in many other publications, and even on several postage stamps. Rob's stock collection can be found here.
The more I look at this image, the more I like it. When we've finalized the design based on this photograph, I'll post an update here.
Updated 2010-01-09: minor wording corrections.