Red Hat Enterprise Linux Deployment Guide; Red Hat, Inc. — The Workstation Security chapter explains, in a concise manner, how to secure the GRUB boot loader.

You can find the current version of the Red Hat Enterprise Linux Deployment Guide at http://www.redhat.com/docs/manuals/enterprise/. In the version of the book for Red Hat Enterprise Linux 5, the relevant section is 43.1.2. BIOS and Boot Loader Security: http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/Deployment_Guide/ch-sec-network.html#s1-wstation-boot-sec.