Following on from Which Call of Cthulhu books do I need to get started?, I would also like to get into Delta Green.
Similar questions, then:
Since I'm familiar with Call of Cthulhu, it would also help me if you could explain how Delta Green differs from it.
Just a Call of Cthulhu rulebook and the Delta Green basic book.
My favorite version of the Delta Green book is the bi-system BRP/D20 one (now out of print). I think the D20 rules for Call of Cthulhu (also out of print, but easily available via Amazon) offer a nice format for playing a more action/adventure themed Delta Green game than might be suggested by using the BRP rules, and there are a couple of nice ideas in there that a solidly BRP GM might find useful too.
Usefulness: The DG sourcebook is probably the most impressive setting book produced for Call of Cthulhu. The wealth of stuff in there is astounding. I don't necessarily think everything the authors did with it follows my own vision of The Mythos And How It Works, but it ain't shabby by any means and I highly recommend it whichever rules you want to use.
Other books: There are other materials produced in the DG line, but they aren't necessary to get started, or to play for months. They refine and expand the materials in Delta Green.
Overview? Think X-Files meets Call of Cthulhu.
The basic book is very useful on its own. It can be used with pretty much any system, as it is high on substance and low on crunch.
Not essential, but useful. The original DG book has conspiracies aplenty and a set of good adventures to get started with.
Agree with Roxysteve's tagline: this is X-Files crossed with Cthulhu.
All you need is a CoC rulebook and a DG sourcebook, but DG: Countdown is also really great work. DG: Eyes Only is not at the same level, I would say. And I don't know yet DG: Targets of Opportunity.
To improve the atmosphere and your background knowledge (and also for fun), you can try the novels and short stories.
It's not out yet but Pagan Publishing has announced they will be releasing a standalone Delta Green game that should still be compatible with existing DG products. This game will also deal with the changes in the real world since the mid-90s and how Delta Green (and the other conspiracies) have adapted/changed. No release date has been given but it's likely to be sometime in 2012 at the earliest. You can listen to the Delta Green panel from GenCon 2011 discussing that here.
Right now, there are four ways to play Delta Green, with a variety of systems:
Call of Cthulhu plus the DG Corebook/dual stat version (BRP)
D20 Call of Cthulhu plus the dual stat version (d20)
Night's Black Agents "Dunwich Sanction" frame plus Trail of Cthulhu (GUMSHOE)
NEMESIS system (ORE). There is nothing particularly DG about Nemesis, except that it was written by Dennis Detwiller with DG in mind.
Another way, which might require some work, is to combine Realms of Cthulhu and Agents of Oblivion for Savage Worlds.
Countdown is a mix of articles about DG setting elements and adventures. Two of the former include Russian and British groups analogous to Delta Green. It's huge book with many templates for international characters and psychic powers too, and the later books follow this format somewhat. Most people consider it better than the DG corebook itself.
Eyes Only has two long articles about the Fungi from Yuggoth & The Fate, a shorter article about post 911 gaming, and 3 adventures. Some of this material was released in a hard-to-find chapbook format at the turn of the century. The Artifact Zero adventure has probably the best reveal in all of gaming.
Targets of Opportunity has five long articles about four Mythos threats & a Canadian alternative to Delta Green. While there are no dedicated adventures, the four bad guy groups are easily enough used as campaign starters.
If you want a look at things right away, check out Dennis Detwiller's blog, which has four free adventures (Music from a Darkened Room, The Last Equation, The Night Floors & A Victim of the Art - these last two appear in Countdown as well). The NEMESIS rules are there as well. Here are the first three parts of his "Future Perfect" campaign. I'm not sure where the final rough-around-the-edges conclusion is.
Delta Green answers two questions which can dog Keepers: "Why would an FBI agent share confidential information with a university professor" and "what the hell has the Mythos been doing since Lovecraft wrote about it?" Its brilliance is answering the latter with modern nuggets of conspiracy theory, like UFOs and the Philadelphia Experiment.
The core book mentioned is from 1997 and it's now been superseded by Delta Green: Agent's Handbook which is a complete rule system for Delta Green, released in 2016.
Also, there's a quick-start primer called Need To Know which is available for free and covers the basics of the rules, so a very nice place to start.
Either of those would be enough to get started playing your first session, so they're very useful.
In terms of other essentials, the Handler's Guide is an extensive history of Delta Green and the world, which is a good resource for fleshing things out.
