MuOS is a 32-bit OS project I work on. Currently no releases or documents, but there will be later!
Intel's Architectural Manuals for the x86-platform in PDF-format:
Volume 1: Basic Architecture - Covering the basic structure of a CPU.
Volume 2: Instruction Set Reference - Detailed description of all the assembly instructions.
Volume 3: System Programming Guide - Covers many topics about protected mode programming. Very useful.