The Little Book About OS Development by Erik Helin and Adam Renberg aims to provide guidance to hackers, tinkerers and developers on the demystification of kernel programming for x86 operating systems, with a particular focus on how to build a system from scratch.
It offers a step-by-step guide to setting up the environment, bootstrapping the OS, handling interrupts, implementing virtual memory, system calls and multitasking, and common pitfalls.
The book is downloadable as a PDF and is also available online.
Another recommended resource for OS development is the OSDev Wiki, which has more information than just x86.
