There is a problem with the code in chapter 7. Page 160 calls for changes to the source code file MythicalPerson.m, which does not exist. I am using XCode 3.2.3 on Snow Leopard 10.6.6, but I believe this problem also occurs on all versions, and it certainly occurs on XCode 4. It seems that there is a step missing in the book.
This blog post explains how to make this code work on XCode 4:
This post explains how to get around the removal of the Core Data Entry Interface Assistant from XCode 4. But the bottom paragraphs of the post are also useful for explaining how to create the MythicalPerson class using XCode 3.2.3 and making sure it subclasses properly. If you're using XCode 3 or if you've already created the GUI but you still need to add this code, the bottom of this blog post has what you need.
Thank you, Jack Nutting, for pointing me to this forum. I will keep looking here in the future for more errata and tips.