I recently bought a book called Developing Your own 32-Bit Operating System.
It was later released by the author for free here
http://www.ipdatacorp.com/mmurtl/So far, Its really good and I would recommend it to other people working on writing an operating system.
Other books I am reading that are useful for those doing OS Development are:
Operating Systems: Design and Implementation 2nd Edition
Design and Implementation of the 4.4 BSD Operating System
80386 System Software Writers Guide
Intel 80836 Programmer's Reference Manual