The primary source for information about high availability under Linux is the homepage of the Linux-HA project (http://linux-ha.org). This contains a wide range of tips and links to documentation, reports and scenarios.
For information in print about high availability see Blueprints in High Availability:
Marcus, Evan & Stern, Hal: Blueprints in High Availability. John Wiley & Sons Inc., 2000. - (ISBN 0-471-35601-8)
There is a very detailed redpaper for Linux on IBM S/390 and zSeries with a wide range of sample scenarios and configurations under: http://publib-b.boulder.ibm.com/Redbooks.nsf/RedpaperAbstracts/redp0220.html.
A Wiki relating to HA and heartbeat: http://wiki.trick.ca/linux-ha/PressRoom and http://wiki.trick.ca/linux-ha/HeartbeatTutorials
The homepage for the DRBD project: http://www.drbd.org/
An article in the Linux magazine:http://www.linux-mag.com/2003-11/drbd_01.html
A detailed collection of links relating to the topic of RAID: http://linas.org/linux/raid.html
The Linux Clustering Information Center home page offers further information about clustering: http://www.lcic.org/
The home page for the Linux Virtual Server project: http://www.linuxvirtualserver.org/
You will find information about the Oracle cluster file system on the project home page under http://oss.oracle.com/projects/ocfs/ and detailed documentation under: http://oss.oracle.com/projects/ocfs/documentation/.