The Linux Kernel Hackers' Guide (the KHG) contains descriptions of many of the functions that you need to use to write a device driver. By the time this article goes to press, a new revision of the KHG should be available on ftp sites, including tsx-11.mit.edu and sunsite.unc.edu.

Barring disaster, the most recent version can always be found at tsx-11.mit.edu in the /pub/linux/docs/LDP directory.