The MP Specification:
Intel PCIsets-Datasheets on the WEB
APIC & IO Chips:
Pentium Pro Processor BIOS Writer's Guide V2.0
Multi-processor locking in 4.4BSD Lite2, by Thorsten Lockert (firstname.lastname@example.org)
[missing in action] X86 Multiprocessing Basics , includes discussion of Open APIC (rumored to be dead, by the way...).