/usr/share/doc/grub-<version-number>/ — This directory contains good information about using and configuring GRUB, where <version-number> corresponds to the version of the GRUB package installed.

info grub — The GRUB info page contains a tutorial, a user reference manual, a programmer reference manual, and a FAQ document about GRUB and its usage.

https://www.gnu.org/software/grub/ — The home page of the GNU GRUB project. This site contains information concerning the state of GRUB development and an FAQ.

https://www.redhat.com/mirrors/LDP/HOWTO/mini/Multiboot-with-GRUB.html — Investigates various uses for GRUB, including booting operating systems other than Linux.

https://www.linuxgazette.com/issue64/kohli.html — An introductory article discussing the configuration of GRUB on a system from scratch, including an overview of GRUB command line options.

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.