If you're learning to build Operating Systems, you'll no doubt have searched and come across MikeOS. It's an open-source (BSD License) Operating System developed by Mike Saunders for assembly language developers to get started with writing their own Operating Systems.
MikeOS has been in development since 2006 and it has only become more active in recent days. The handbook accompanying the source code walks you through every bit of the code and also explains the reasoning behind the code structure.
MikeOS is quite capable is a starter's Operating System - it supports interaction with the serial port and the PC speaker, the FAT12 file system, and DOS-style interrupt handlers.
You can go to the MikeOS project website for more information at http://mikeos.berlios.de/