Hacker's Databooks for the FreeBSD SMP kernel

Intel Databooks:

Pentium Processor Family Developer's Manual:

Vol 1: Pentium Processor, order# 241428
Vol 3: Architecture and Programming Manual, order# 241430

Pentium Pro Family Developer's Manual:

Vol 3: Operating System Writer's Manual, order# 242692-001

Intel486(TM) MicroProcessors and related Products, order #241731-002

pp 4-220 thru 4-302: 82489DX Advanced Programmable Interrupt Controller

Pentium and Pentium Pro Processors and Related Products, order #241732-003

Data sheets on CPUs, motherboard chipsets, etc.

Peripheral Components, order #296467-007

Motherboard chipsets, etc.

Other useful books:

Pentium Processor System Architecture, 2nd edition

PC System Architecture Series, MindShare Inc.
ISBN 0-201-40992-5
includes a 45 page chapter on the APIC.