Delta Green: Agent's Handbook is the "Players Guide" for the standalone version of Delta Green, and must have if you want to set a Delta Green game. This book contains the rules for the game, the complete character creation, a chapter with profession options from various American government agencies (extremely well-researched by the way), and "Tradecraft", more or less a guide for how to players act like spies.
The standalone Delta Green is a fantastic system, the themes and setting of the game work very well. I'd say it can be darker then standard Call of Cthulhu, it spices up that formula with paranoia, the dissolution of daily life, the apocalyptic dangers that are ever more present and just the general dickness of mankind. It's the great irony of DG: You might have big guns, but that isn't enough to defeat them, it will never be. As a side note, I like how the Agent's Handbook doesn't contain actual Delta Green lore, this serves well for two purposes: That the players not knowing the actual threats and history of the organization can make the general sense of paranoia and mystery better ("who are those guys", "who we really work for", etc) and fixes the problem from the old DG books where players and Keeper stuff were in the same book (after the keeper-only chapter), so no more "oops, accidentally read the whole chapter on the bad guys"), the other is that the amazing rules (covered below) can be used for more than playing as creepy Man in Black agents.
On rules: It's Call of Cthulhu, but better. The DC is the skills in your sheet: roll below them, they have been reorganized for the more modern setting. Combat is made more tactical, fast-paced and chaotic, almost as an actual firefight in real life, initiative is now whoever has more dexterity goes first, and the new "lethality" rule makes stuff faster and deadlier:. If you roll the "lethality" in that range, the weapon just kills the target instantly. The new Sanity system is a bit inspired in Unknown Armies: You roll if you character experiences "violence", "helplessness" and "the unnatural", and you can "adapt" for the first two by rolling bad three times (but never the unnatural). You can gain mental ilnesses (PTSD, Alchoolism, etc) from those encounters. The real meat is the new "bond" system, your character has a few bonds which are family, friends, coworkers, and each one has a score, you can make SAN loss less worse in exchange for your relationship with the bonds, so your PCs relationship with friends, family and will get worse and worse as you become more alienated and your life desintegrates.
There's also new requisition rules, for getting stuff, more more simplified in relation to CoC.
If you want to play a horror game, this game is for you. If you are a seasoned (hell, or new) CoC player, this game is for you. Absolute recommend for anyone trying a new and spooky game.
[5 of 5 Stars!]