Don't know if these are "best," but they're good and available:

Complete Reference: Bash Reference Manual: http://www.gnu.org/software/bash/manual/bashref.html

Complete Reference: Harder to find things but you should know your way around it: $ man bash

Intro, Fundamentals: BASH Programming - Introduction HOW-TO: http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html

You'll get here soon enough: Advanced Bash-Scripting Guide: http://www.tldp.org/LDP/abs/html/

Ideas:

http://www.bashoneliners.com/

http://www.commandlinefu.com/commands/browse

http://www.catonmat.net/sitemap/