I am trying to learn how to develop drivers for linux. I have been programming in user space for some time, but I have wanted to get into kernel land for awhile.
However, right now it looks to me as if there is no documentation whatsoever on this subject. So, everything I am learning is from drivers developed by others. (So, if anybody has any helpful links to documentation, that would be great)
Anyway, I'll get to the point.
when I run the command "lsmod" I get a list of all the modules loaded by the OS, and I notice that some modules are "used by" other modules ('usbhid' uses 'hid'). How does this work? How do these modules communicate with each other?

You may wanto to have a look at this.

Cool, thanks. That looks very helpful.
I looked for a book before, but everything I found seemed to be way out of date (like for 2.4 kernels).