Earlier in my life I did a lot of Windows system-level programming, including some minor driver development, and now I'm practically exclusively on the #nix bunch, sometimes doing similar stuff.
I can't say that there is a disparity. If anything, the parts that are documented are usually better documented in the Windows world, and there are very good books about most components. (To be fair I never read a book on anything unix apart from Lion's). There is a large amount of stuff online for either, but I often have the feeling that correct examples are less frequently found for, esp., Linux.