Season of Skulls came out six months ago, so here be spoilers. If you want a recap of the two previous novels in this not-exactly-a-trilogy, you can find my notes on them here: Dead Lies Dreaming, Quantum of Nightmares.
What do I mean by not-exactly-a-trilogy? Well: the idea of the New Management was to reboot the Laundry Files, which I've been writing since 1999, with a new cast of protagonists largely drawn from a younger generation, and dealing with more modern social and political issues. My little one-shot Lovecraftian-spy mashup has evolved over two decades into a sprawling setting with multiple viewpoint characters, but they were all essentially civil servants working in the secret side of the government, which is a bit restricting. I blew the doors off the universe in The Nightmare Stacks, allowing me to explore the overarching theme of how to live in a world gone mad—the world of the New Management—and this trilogy marked the start of that. Also, the Laundry Files main story line comes to an end in late May of 2015, in an event that can reasonably be called the Lovecraftian Singularity: the climactic events, from The Nightmare Stacks onwards, are all crammed into a period of about 18-24 months. 2015 is receding in the rear view mirror and I wanted to jump the the setting forward a bit, so Dead Lies Dreaming starts in winter of 2015 and Season of Skulls takes place in spring of 2017 ... and, of course, the summer of 1816.
(I have plans for more New Management novels, starting with one in 2019, but my editors are holding my feet to the fire and insisting I finish the earlier series before I go there. And they're right. So I'm working on the final installment in Bob and Mo's story arc at the moment ...)