Hardcore Coding

I've never really had the urge to hack on the Linux kernel (not least because I am the world's worst programmer - Fortran, anyone?) but if I did, I'd certainly be using Greg Kroah-Hartman's Linux Kernel in a Nutshell. To both his and O'Reilly's credit, you can download a copy (cc licence), but obviously buying one would be a good idea, too, for all the obvious reasons.