If you have a Raspberry Pi, you might consider reading http://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/ Note though that the one for the Pi only scratches the surface. Topics like memory management or task scheduling are not included.
http://pdos.csail.mit.edu/6.828/2012/xv6.html -- read thru src code of unix v6, polished for x86 as xv6
Once done with xv6, http://zoo.cs.yale.edu/classes/cs422/pios#! -- Build a modern multi-core os kernel