Brief Overview
There are multiple sources of information for Delta Green, and most of these are common knowledge if you're already a fan of the game. But I thought I would compile them all into a single post to get things started.
Lastly, if you're looking for a layman's description of what the setting is like, it can best be described as a conspiracy within the US Government willing to use extra-judicial means to keep the world safe from unnatural threats. There are other-national offshoots, there are conspiracies within conspiracies.
I always describe it as, Men in Black if it were a violent, R-Rated crime-drama or X-Files if it were all about the Cigarette Smoking Man.
While not a DG inspired series, as far as I'm aware. True Detective Season 1 starring Matthew McConaughey and Woody Harrelson is an excellent, engaging primer on how the stories hit.
Links to the creators/maintainers
This may not be comprehensive, but the following people either have/had a strong hand in the creation of the Delta Green Lore, or are working to maintain/embellish it.
Dennis Detweiler patreon
Adam Scott Glancy
Shane Ivey patreon
Caleb Stokes patreon
John Scott Tynes website
Podcasts
The Greenbox Podcast
The Rancor's Brothel *Site was down when I tried to post this, but they have several episodes set in the DG mythos
RPPR Actual Plays featuring Delta Green
Handler Tools
The Fairfield Project Fairfield Project is a collection of "ideas" for Delta Green Handlers. There are everything from objects/items, shady groups, and shotgun scenarios available here to boost one's creativity. Most of this information revolves around "The Outlaw Years" of Delta Green (a point in time in which DG was officially disbanded, yet the agents were still operating very much outside the law.
"The Delta Green Wiki" or "The New Fairfield Project" Similar to The Fairfield Project, the Delta Green Wiki is an offshoot wiki that is primarily focused on the timeline after DG has reformed as a branch of the government with slightly better access to resources than the Outlaw Years.
The Green Box Generator A "Green Box" is a storage cache (historically a nondescript storage unit) of items that would be useful to Agents when going through a scenario, and provided by the Handler as a means to provide them with interesting/useful/quirky/dangerous artifacts. The Green Box Generator aids a Handler in creating a list of interesting items to populate a Green Box with.
The Scenario Database Maintained by our own @blaborb this is a fairly comprehensive database of of 649 (at the time of this writing) DG scenarios in print/out of print/shotgun scenarios/etc. with tags, voting and links.
Kickstarter Reward Report A link from Arc Dream's main website where they are keeping a loose record of what is released and what's on the horizon. No exact dates in most cases, but they do try to give you an idea of how far along each project may be.
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This was my first attempt at starting a post to the community. I would love to add more to this, and enrich/maintain better links as time goes on. Please feel free to update or correct me in the comments, and I will make an effort to expand upon this post. -Thanks!