This covers warfare on two very different planes, the kick-in-the-door-and-shoot-the-tentacled-horror-from-beyond-space heroics of Special Forces, and the agenda-to-agenda conflict in committee rooms that decides what the Laundry - and other parties - want to get out of the mission.
The Special Forces section allows you to create characters who are SAS or SBS (or any other part of UKSF - the UK's Special Forces directorate), who can turn up when the amateurs from the Laundry mess things up and put out a call for OCCULU support. There's a fairly strong suggestion that it makes sense for players to have two sets of characters, their mainstream Laundry characters and the knuckle-dragging doorkickers. The whole section is well written, well informed, and occasionally laugh out loud funny (Some of the humour is likely to pass you by if you aren't British, and possibly of a certain age. For instance, SAS slang for magic is 'animal', and that's never explained. But Brits like me will be sniggering over the reference to 'Animal Magic', a kid's TV show baack in the 70s. There's not a huge amount of this, but it is there.). And it doesn't just stop at character creation, it looks at how occult operations might be different from conventional operations - such as using radar-altimeter equipped 40mm BATSTOP rounds to set up a mid-air no-go area for flying horrors, or why in a conventional operation you would want to be in a ditch during a firefight, but when taking on a horde of shambling zombies you would want to be behind it. This is really well thought out.
Next up is an SF scenario, On Borrowed Time - an alien installation just appeared next to the UK Jungle Warfare training base in Brunei, and the last message from the Laundry team sent to investigate was that they were under attack, so the players get to airdrop in to find out what happened to them, and kill it. It's not bad, but there's a strange mix of quite simple shoot-it-now and we-need-to-think-really-carefully-about-this.
And then we're back to the green baize tables of Westminster committee rooms. The basic Laundry Files includes a really well thought out mechanic which gives you a budget for missions, which can be augmented by the use of your Status (a character stat) within the Laundry. So while it is possible to exceed your budget and call in OCCULUS support (or whatever) for every mission, the downside is that you will then find your supervisor complaining loudly when the bill for said OCCULUS support (or whatever) comes out of their budget. Which can mean not getting sent on that Computational Demonology training course your character really wants to go on. It's really neatly self-balancing, you can abuse your budget, but there will be consequences. And it really fits the bureaucratic ethos of the Laundry.
As Above, So Below extends this mechanic. You now can call on more than just your own Status stat, you can draw on your department's Status, plus that of your mentor, if you have one. And the section runs through a whole range of mentor types, from Deeply Scary Sorcerors, to MPs and Ministers (dangerous), the opposition (MI5 and MI6 that is - very dangerous), to journalists (flee, you fools!). And if your Status grows high enough, then you too may be promoted to management and expected to sit on the committees that decide Laundry policy for the next expedition to the Plateau of the Sleeper, or, more likely, the precise wording to the latest amendments to standing procedures for paperclip audits.
Putting players on high level commitees for operational policy is the main focus, and the suggestion is that they should again have a parallel set of characters, each with their own agendas, whether departmental or otherwise. Which is fine, not sure I'd want to roll it out a lot, but worth playing with from time to time. What I'm not sure about is whether I like the mechanic used, which uses a set of playing cards, which are given values for agendas and issues, with the idea being to trump other people's agendas and issues, possibly in combination with other players. It sounds workable, and I may just be having an irrational reaction to the playing cards, but cards aren't used anywhere else in the game. I think I'd be tempted to rewrite it to something closer to Status if I wanted to play it repeatedly.
And the last part of the supplement are two more scenarios, the first with the players themselves on a low-level committee, and the second using the dual level committee play mechanics. In Hot Potato, the characters end up one one of those committees everyone tries to avoid, in this case setting up the government bunkers for CASE NIGHTMARE GREEN, but something's gone wrong with a Compulsory Purchase Order (USAians think eminent domain) and suddenly the press are all over it, when they shouldn't even know the Laundry exists. There's a leak in the committee, and the players are on a mole hunt. But this mole may be burrowed much deeper than they imagine. In Fire Drill, a Laundry agent sent on a mission to Kazakhstan disappeared several months ago, but has now been caught on the dashboard camera of an embassy car. The committee has to decide whether to focus on recovering him, carrying on his investigation (cases of spontaneous human combustion, possibly linked to a Russian rocket company) or whatever, a decision which the agent characters then get to implement it. There's a lot of flexibility built into this, possibly too much, the threat can be anything from the odd spontaneous human combustion, to world threatening. I think it might have been better to take a little of the flexibility of the threat out in order to concentrate attention on the flexibility of the mission planning - which is what the scenario is supposed to be showcasing. Which is not to say it's a bad scenario, just that it may be trying to do too much at once.