Buy it!  Don't search for e-books!

Solaris Internals: Core Kernel Architecture by Jim Mauro and Richard McDougall.

Mac OS X Internals: A Systems Approach, by Amit Singh

UNIX Internals: The New Frontiers by Uresh Vahalia

Title: Understanding the Linux Kernel
Third Edition: November 2005
ISBN 10: 0-596-00565-2
ISBN 13: 9780596005658

Linux Kernel Development by Robert Love, second edition from Novell Press.

Title The Linux® Kernel Primer: A Top-Down Approach for x86 and PowerPC Architectures
Authors: Claudia Salzberg Rodriguez, Gordon Fischer, Steven Smolski
Publisher: Prentice Hall
ISBN 0131181637
Year 2005
Pages 648