Eldar Tactics:

https://www.goonhammer.com/9th-edition-faction-focus-craftworld-eldar/ Goonhammer is a great resource for tactics, unit reviews, and more. The link above goes directly to the article on the Craftworlds faction as a whole, but you can find lots more useful and entertaining content if you take time exploring the site. Although Goonhammer has several authors, content is continuously “voiced” in a way that is accessible to new players, clear, and engaging. There is no better place online for in-depth picture tactical advice for 40k.

https://www.frontlinegaming.org/tactics/ Frontline Gaming is a potent force in the 40k community. They run the ITC,(a prominent series of big tournaments, as well as providing helpful resources for players. (Scroll down for the eldar pages.) You should also check out the Frontline Podcasts, some of which are interviews with some of the most competitively successful eldar players; they share lists and talk tactics. 

https://www.youtube.com/watch?v=OXzeFg5q7uQ
This video was one of the first tactics resources for eldar players after 9th edition appeared. These guys call themselves The Tabletop Titans, and they exploded onto the 40k internet scene by releasing “how-to” videos for pretty much every faction in the game in a very short time period. 

https://www.facebook.com/splintermindpodcast/ My favorite eldar podcast. It addresses all the Aeldari factions, but there are lots of useful episodes for Craftworlds players. Also, unlike lots of amateur podcasters, the primary host has “radio voice,” which the show easy-listening.

Lore:

https://www.youtube.com/watch?v=Ya660UG4i9o (A great video on the origins of the Eldar)

https://warhammer40k.fandom.com/wiki/Eldar (A wiki page where much eldar lore from the various sourcebooks over the last few decades has been consolidated into one place.)

https://www.reddit.com/r/40kLore/comments/8q3pg9/eldar_things_people_ignore/ This article is dedicated to stuff even eldar players don’t know about the eldar. It’s excellent. Informed and insightful; it showcases the depth and complexity of the different craftworld civilizations.

https://gavthorpe.co.uk/2017/07/05/eldar-qa-part-three/ (Gav Thorpe, author of the eldar path trilogy, answers questions about eldar society and history)

Rules:

http://wahapedia.ru/wh40k8ed/factions/craftworlds/ (All the rules for every faction in one place.)

https://www.battlescribe.net/?tab=news (An application for easy list-buidling)

General 40k Tactics:

https://www.goonhammer.com/start-competing-strike-force-missions-in-9th-edition/ (A non-eldar specific guide to 2k matched play in 9th edition) This is another of the MANY incredibly useful articles on the Goonhammer site.

Getting Serious About Tournament Play:

https://www.goonhammer.com/start-competing-40k-practice-part-1-prepping-for-your-first-tournaments/
This is the first of three excellent articles from Goonhammer about how to move from being a casual 40k casual 40k player who gets in maybe a dozen games a year, to a player who can reasonably hope to win some games at one or more of the really big tournaments. These are must-reads if you have ambitions to attend one of the big opens.

9th Edition Craftworlds Battle Reports:

Craftworlds vs Tau: https://www.youtube.com/watch?v=hDc-45puE0M
Craftworlds vs Adeptus Mechanicus: https://www.youtube.com/watch?v=xVHP4PnOER4
Craftworlds vs. Dark Angels: https://www.youtube.com/watch?v=oMvbCdVoCGM&t=8077s
Craftworlds vs Space Wolves: https://www.youtube.com/watch?v=G5uEDvS1RyI
Craftworlds vs Khorne: https://www.youtube.com/watch?v=dLzNkBB5Cjo

Painting:

If you want to recreate the color scheme I use: https://www.youtube.com/watch?v=KjuBsBFMtl4

A guide specifically for painting soul stones: https://www.reddit.com/r/Eldar/comments/f60gjp/my_quick_n_easy_gems/

Magnetizing:
Intimidated by magnetizing your units in order to swap weapon load-outs? Don’t be. This Reddit Hero has created a fantastic guide for magnetizing harlequins that works for anything: https://imgur.com/gallery/ZLarjVC  

Commission Painting:
I have always painted my own models, with the exception of the occasional ebay find that is good enough as is; however, my wife and I recently had a baby, and so I broke down and sent an inquiry to a guy in Italy about painting a Harlequin force that I knew I would never get to between the new baby, and all my other Warhammer projects. I was shocked when he came back with a super affordable quote. Not only did he do a great job, but he finished 35 infantry, 3 transports, and 6 bikes in just a few weeks. I could not recommend this dude more highly: https://yourpaintingla.blogspot.com/?fbclid=IwAR0UDPhv1xxu8B2dv3m-gpa17tt_4sUFhGxg-L1DSnC96GlWRy6b1RNpsrg

And if you are thinking you commission painting seems like an unjustifiable luxury, it is probably a better use of funds than buying more unpainted models when you already have tons of unpainted models that you are unlikely to get to.

An Aeldari-Themed Battle Mat:

This high quality gaming mat from Frontline features overgrown ruins that could pass for an Exodite Colony, a Crone World, or Even an ancient and disused part of a Craftworld. It is the best neoprene eldar table mat that I know of: https://store.frontlinegaming.org/flg-mats-ancient-alien-ruins-44-x-60.html

For Collectors:
White Dwarf 127: Introduced Craftworld Eldar as we know them today. Aspect Warriors, Aeldari Gods, The Doom, all of it was new in this issue of WD, and I am not sure it has been so eloquently explained in any official GW publication since. If you can find a copy on ebay, it is a delightful edition to any collection. There is an enormous dragon on the cover of the issue.

White Dwarf 236: Introduced of new Aeldari models and has a great article on how rules designers thought about the faction early on. There is also a great battle report in which the now-famous but then fresh-faced Gav Thorpe throws down against marines. 

The very first Eldar Codex, (2nd Edition 40k) This book introduced lots of lore, some new units, and contains some of the coolest and most influential Craftwrolds art of all time. It also has rules for exodite armies that ride dragons and pirate armies. Classic.