I was looking for something related to the linux kernel, went to fall in ibm website displaying these two documents, written almost three years ago, introducing some very basic idea of the linux 2.6 kernel, two sets of around 20 pages each, interesting introductory material:

Hacking the Linux 2.6 kernel, Part 1: Getting ready

Hacking the Linux 2.6 kernel, Part 2: Making your first hack