To read The Linux Kernel Hackers' Guide, you should have a reasonably good understanding of C. That is, you should be able to read C code without having to look up everything. You should be able to write simple C programs, and understand struct's, pointers, macros, and ANSI C prototyping. You do not have to have a thorough knowledge of the standard I/O library, because the standard libraries are not available in the kernel. Some of the more often used standard I/O functions have been rewritten for use within the kernel, but these are explained in this book where necessary.
You should be able to use a good text editor, recompile the \ kernel, and do basic system administration tasks, such as making new device entries in /dev/.
You should also be able to read, as I do not offer support for this book...
``Hello, sir, I'm having some problems with this book you wrote.''
``Yes?''
``I can't read it.''
``Is it plugged in?''
``Yes. I also tried a lamp in that socket, so I know it is getting power. But I really don't think that's the problem.''
``Why not?''
``I can't read.''
`` Oh. Well, let's start here. See this? Repeat after me: The cat sat on the rat...''