The OpenBSD website, http://www.openbsd.org
The FreeBSD website, http://www.freebsd.org
The NetBSD website, http://www.netbsd.org
The Linux Documentation Project website, http://www.tldp.com
The GNU HURD Project website, http://www.gnu.org/software/hurd/hurd.html
Cisco Systems website, http://www.cisco.com
The SCO Group website, http://www.sco.com
Apple OS X website, http://www.apple.com/macosx/
The Apple Darwin Project website, http://www.apple.com/macosx/features/darwin/
The IBM AIX operating system website, http://www-1.ibm.com/servers/aix/index.html
Hewlett-Packard HP-UX website, http://www.hp.com/products1/unix/operating/index.html
The SUN Solaris 9 operating system website, http://www.sun.com/software/solaris/index.html
Everything Solaris Repository website, http://www.everythingsolaris.org
Solaris freeware website, http://www.sunfreeware.com
Applied Operating System Concepts, by Avi Silberschatz et al. (Wiley Text Books, 1999).
Distributed Operating Systems, by Andrew S. Tanenbaum (Prentice Hall, 1994).
Distributed Systems: Principles and Paradigms, by Maarten Van Steen and Andrew S. Tanenbaum (Prentice Hall, 2002).
Modern Operating Systems, Second Edition, by Andrew S. Tanenbaum. (Prentice Hall, 2001).
Operating Systems: Design and Implementation, Second Edition, by Andrew S. Tanenbaum and Albert S. Woodhull (Prentice Hall, 1997).
Operating Systems: Internals and Design Principles, Fourth Edition, by William Stallings (Prentice Hall, 2000).
Inside Cisco IOS Software Architecture, by Vijay Bollapragada et al. (Cisco Press, 2000).