You can find a copy of Linux Device Drivers, 3rd Edition on LWN. The work, current to Linux 2.6.10, is licensed under a Creative Commons Attribution-ShareAlike license. While it’s available for free, if you really dig it, grab a physical copy published by O’Reilly to support the authors and forward-thinking publisher.
I’m definitely not a hardcore kernel hacker, but it’s great to have this resource available in a form that I can graze on if I wanted to get my feet wet. Thanks so much to Thoron on #maemo for pointing this out.