Every World of Warcraft raider should read these guides and check out these tools. They are timeless, more important than any class guide, and they apply to every class. Class guides teach how to play a specific class; that will change from patch to patch. These guides and tools teach how to raid, and that knowledge will endure for your entire raiding career. If you know of a good guide, suggest it!

Raid Awareness

(Raid Awareness is a Learned and Practiced Skill): Staying alive is the most important skill a raider can have. If you are going to read one link on this website, read this one.
(http://www.paragon.fi/blogs/rng-not-excuse): There is little randomness in wow that can’t be dealt with.

Logs

http://www.icy-veins.com/forums/topic/4392-warcraftlogs-a-comprehensive-guide/ How to take logs and upload to warcraftlogs.
https://wowanalyzer.com/ - This site will take your warcraftlogs parses and and analyze them for you.
https://www.reddit.com/r/CompetitiveWoW/comments/3sqix0/resource_compiled_class_metrics_for_log_analysis/ - How to analyze specific classes in patch 6.2
http://us.battle.net/wow/en/forum/topic/15538506318 - how to take logs and analyze Warlocks in 7.0
https://sacredshielding.wordpress.com/2016/05/12/hpal-log-analysis-101/ - This guide is specific to holy paladin for patch 6.2, but you can apply the concepts used here to your own healing, dpsing, or tanking.

Gearing/SimCraft

https://github.com/simulationcraft/simc/wiki/StartersGuide: Simulationcraft is a program that simulates combat to generate stat weights and trinket rankings that guide gearing choices. The above link is their introductory guide.

UI/Addons

(https://www.reddit.com/r/maximumcharactercoun/wiki/wow_curated_addons_list) Curated addons list compiled by redditor http://www.reddit.com/u/maximumcharactercoun.

Weak Auras

(http://www.mistyteahouse.com/weakaura-hub/weakaura-school/) A guide to weakauras, a powerful addon that allows you to monitor most events in the game, including your abilities’ cooldowns and charges.

Getting in a Guild

(https://www.youtube.com/watch?v=uieU0KOIoPw) This video guide by Mike Preach explains how to get into a (better) guild. He speaks a bit loudly.
(http://www.peakofserenity.com/finding-a-guild/) This written guide by Babylonius explains how to find the right guild for you and how to go about the search.