If you're in school, Microsoft also has an academic program that allows universities (almost) full access to the source code of various Windows kernels. Its called the Windows Research Kernel program and might help you along if you can get ahold of it because it comes with a ton of documentation: the original NT design papers, proposed class materials and a full copy of the aforementioned Microsoft Windows Internals book in PDF form.
For Mac, the online kernel guides are pretty good. The Mac OS X Internals book isn't so great for driver or kernel development directly, but is a good reference for learning about how things work. Having said that, the book is a bit dated at this point and could use an update for the later kernels.
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