2 topics that I think are usually glossed over in the other books are multithreading and building custom panels.  Maybe not appropriate for a beginner book, but their absence keeps the other books as mear academic discussions rather than something useful in real world apps.  Would love a book that focused squarely on those topics.

Color code samples, like Adam Nathan's

Mr. Eisenberg,

Perfect timing. I was looking for a WPF windows client book for beginners and came across your blog.  The WPF books I've seen to date do not provide a complete real world working example.

Simple analogy - you teach someone how to use a hammer,

screwdriver, saw , chiesel, drill etc. Now it's time to build something and call it your own !  

Perhaps Will's suggestion could be addressed in a second book.

Why limit yourself to writing just one book ? As for Connor's comment - color is good - except if you're color blind.

I'm in the middle of writing a book on Silverlight for designers and developers and I'm not sure its worth the money.  I have talked with a lot of authors that seem pretty worn on the whole process.  I'm not sure how far you are so far but I hope it goes well.  For me I think its more or less a labor of love.  and as to suggestions.  For me I like things in small simple chunks that are self contained and work.  both of these have been issues with books before that I have picked up...  (and put down)