Cthulhu Deep Green is a 52 page espionage horror game that splits the difference between Cthulhu Dark / Delta Green.
It has solid layout and artwork, it's very readable, and it steers in a slightly different direction than the modern Delta Green books---keeping the same overtones, but making a choice on whether to focus on storytelling or crunch.
The mechanics are simple but have some meat to them. If you're not familiar with Cthulhu Dark, you roll a d6 and on any result, you succeed. 1--3, it's with a complication. Higher than that, it's as you intended, or with an advantage. You can add more dice if your profession applies, and you can add more dice if you're willing to risk your mental state and physical safety.
This emphasis on failing forward makes Deep Green a game where your PCs can have agency, and where they're likely to find the heart of a given mystery---even if they don't overcome it---as opposed to Delta Green's tendency towards one minor misplay obliterating the agents before they know what they're dealing with. I admittedly like both styles, but Deep Green is definitely better if you want to decrease the overall randomness in your stories.
That said, combat and sanity are handled in a bit of an odd way.
With combat, any time you describe a course of action leading to violence where the outcome is uncertain, you fail and are killed. You can choose to rewind time and try to avoid violence, and in which case there's only a 50% chance that you are killed. Granted, the GM can simply wound you instead of killing you, but it means you have to do a *lot* as a player to make sure the outcome is never uncertain if you want to oppose other violent characters or monsters.
Sanity, meanwhile, is handled through the accumulation of stress. Stress builds up when you risk your mental state on rolls, and if it gets high enough, you take a disorder. Disorders can be treated, and agents are way more mentally resilient in Deep Green than in DG, but they also have a separate track, Insight, which progresses them towards oneness with the Mythos and a dramatic exit from the game.
Both mechanics are interesting, but I think I'd urge the GM to be lenient with situations involving violence. Make 'minor injury' your go-to when a PC gets in a fight, and save the 50/50 deaths for when they try to fist-fight Hastur.
Deep Green also has something similar to DG's downtime and bonds system, with the catch that Deep Green's works. Instead of your actions between operations mostly being useless / a way to showcase your descent into personal tragedy, almost nothing's random in Deep Green's At Home time. Restoring a bond just works. Fixing a disorder just works. Lowering your Stress doesn't instead sometimes raise your Stress or have the GM assassinate your PC. It's a breath of fresh air.
Setting-wise, there's very little distance between Deep Green and DG, and Deep Green could be used to play basically any DG scenario. The few hints at difference (terms like 'illuminated', and an actual rulebook for agents) are nice, but I found myself wishing there was a little bit more to set Deep Green apart.
Fortunately, Deep Green comes with an 18 page supplement, Food Of The Gods, that partially scratches that itch. Food is a solid, low-lethality scenario that can still scale up to challenge seasoned players. Some GM interpretation might be needed for how one of its key elements works, and the GM may need to prompt groups that get lost, but Deep Green's fail-forward approach should keep the players pointed straight at the heart of the adventure.
Overall, if you like shows like True Detective and X-Files, if you like Delta Green, if you enjoy Call of Cthulhu or Cthulhu Dark or Trail of Cthulhu or Night's Black Agents, or if you just want to play a tense, immersive game of modern occult conspiracies, there's something for you here---although whether it's your thing or not is going to depend on how much crunch you need. For me, the simplicity of the dice added to the intensity of the game, and I would strongly recommend it to anyone who's even a little bit interested in picking it up.
Minor Issues:
-Page 16, first para, "Burning out" Out
-Page 24, Specialty Skills feels like it breaks verisimilitude a little bit, with getting scuba training making you unable to set bombs, or vice versa. Maybe picking up Specialty Skills could be an At Home action?
-Page 23, there isn't any indication for how to mechanically remove ticks from the exposure clock. Would At Home actions do it?
-Page 47, the font randomly changes here. Intended?
-Food page 10, "(it like honey and fish)" smells like?