The Atrocity Archives was the first Laundry... novel? It's a novel, a novella, and an essay by Charles Stross. The short story is Concrete Jungle, and the essay is really about the influences on The Laundry Files. So I'm going to talk primarily about the novel itself.
The novel is first and foremost an introduction to the setting. It follows Bob Oliver Francis Howard (It's time to play Spot The References, because there's more than one in that pseudonym) from lowly IT technician with attitude to lowly field agent (with slightly less attitude) for the UK's ultra-classified secret agency of the occult, The Laundry. This is less a leap than you'd think: the Many Angled Ones lurk at the bottom of the Mandelbrot Set and you can generate resonance in the fabric of the multiverse by doing the right (or rather, wrong) sort of math. And computers are very good at doing math veeeeery fast. So when a recent university graduate thinks they've found a cool new way to draw some pretty fractals, Wolverhampton may be well on its way to getting a fresh landscaping of demons. At which point the Laundry steps in and stops them, giving the poor sod a Job Offer They Can't Refuse.
This is a pretty cool premise, and it's worked very well for the Laundry for a number of years now. A lot of people have noticed the connection that sorcerers and hackers are more or less the sci-fi and fantasy equivalents of one other, but Stross makes it all feel real with a think layer of workplace banality: Sure, The Laundry's an occult intelligence organization, but it's also a government bureaucracy and an IT organization, and Stross writes it like he's had ample experience with both, and with the kind of people who would work in either. Everything is standardized, there's an eternal stack of paperwork for everything, and you'd better keep the receipts for all your expenditures in the process of saving the world. Meanwhile, when Bob isn't saving the world, it's his job to diagnose ailing Beowulf clusters and fix computers for those bereft of a clue, an aspect of his job that he treats with as much relish as any real systems type would—none at all.
That mix of dark horror and banal drudgery would stick The Laundry Files, arguably intensifying over time, but it starts strong here.
As a returning reader, I'd forgotten just how well The Laundry sets up its supporting cast early on. Ex Girlfriend From Hell and walking Checkov's Gun Mhari Murphy is as well established as she needs to be (it'll be a long time before she comes to fore), Derek the "Accountant" suitably lampshades his real job long before we go in-depth on what it is, Pinky and The Brain are suitably insane roommates who are a bit like if your roomates with crazy and dangerous side projects started dating, and Bridget and Harriet are eminently hateable bureaucrats even when they do have a point. And those characters are by and large not going to be relevant for a long time, if ever, to the point that I'd forgotten some of them were even in this book. The relevant cast (Alan, Andy, Angleton, and Mo) are all suitably memorable and stick with you, which is good because you'll see a lot of them. And of the lot it's clearly Angleton who steals the show: The terrifying boss who seems to be more than what he appears, one part schoolmaster to two parts force of nature. We'll find out more about him as the series continues, but he makes a strong and immediate impression.
Mo, sadly, is a bit weaker in this novel. As a recently inducted Laundry member, she's mostly serving as confused newcomer, quasi-love-interest, and part time Damsel in Distress. As the novels progress, she'll play a larger role and become a force in her own right, as well as developing into a much more interesting character. But for now, she's more side character than anything else. This will not last.
I'm quite a lot down and I haven't talked much about the horror aspects. So I really should because this is... maybe still the most viscerally scary the Laundry gets. It draws on Nazi occultism and the Holocaust for truely gruesome effect, and the climax has all the Lovecraft you could hope for, save that the world isn't destroyed. There is something viscerally haunting about this book's imagery that is rarely captured. The series wouldn't be this dark again until after The Rhesus Chart, or arguably even until The Delirium Brief. I'm not going yo say much more because I don't want to spoil it.
Is there anything bad I can say about this book? Well... maybe. The other thing I forget about was just how in-jokey early Laundry could be. The later books are more serious about this, but for the first two books, and the short stories from that era, the books had a deep atmospheric steeping in geekery, with side jokes about Sendmail rules, Knuth Volume 4, an early nod to Symbolics, complaints about software licensing audits and Quality Assurance, and a collection of dialogue with some clear nods to Ye Olde Jargon File. These elements flaked away over time as the focus became more rooted in story, even if they never totally vanished (there was a great bit about the Laundryverse's Modesty Blaise expy writing in a particularly Lisp-ey dialect of Old Enochian), and it's jarring to see them at near full-force here. Not quite total full-force, because the second Laundry novel included a short story called "Pimpf" which is more or less a send-up to the BOFH (the second novel was a lot more light-hearted, albeit largely for in-universe reasons).
Overall though, I'd highly recommend the book. All the extra references are largely ignorable and the rest of it works really really well.