I read this in the wrong order, in a sense, in that it was a couple of years ago that I read the sequel, The Jennifer Morgue (in which I even get a small credit in the acknowledgements). And I had read the second, shorter, section of this book, "The Concrete Jungle", in the run-up to the 2005 Hugo voting (which of course it won). But the first 250 pages, plus Ken MacLeod's introduction, plus Charlie's afterword on the common features of spy and horror fiction, were all new to me.
Having said that, I still like "The Concrete Jungle" best of the Laundry stories. If I had to choose a single word to describe Charlie's writing, I think that word would be "unrestrained". It's not easy to balance that instinctive narrative style with the bathos required to tell stories of civil servants tasked with fighting eldritch horrors from another dimension, and "The Concrete Jungle" is where he succeeds best. Which is not to say that the main chunk of "The Atrocity Archives" is bad, far from it - there are some memorably creepy moments, such as the death of Fred from the accounts department and the exploration of a frozen parallel Earth - it's just that the Hugo voters got it right, as they sometimes do.
And the Ken MacLeod intro and Stross postscript are worth reading too; indeed, the postscript was the only point where I really regretted not having read this before The Jennifer Morgue, as the epilogue to that book seems like a continuation of the same conversation between author and reader. Having said which, this author is one with whom this reader has little difficulty in conversing.