For Linux, you might look into picking up the O'Reilly Linux Device Drivers book or reading PDFs online. In my opinion, it is one of the better texts around on the subject.
The Linux Kernel Module Programming Guide is another good resource.
You may also want to pick up a book specifically on the Linux Kernel. I picked up a copy of Robert Love's Linux Kernel Development (2nd Edition) for this purpose (3rd Edition on the way).
