Since I'd read (and absolutely loved) the previous book set in this universe, Archivist Wasp, I was really pleased to hear about Latchkey and while for me it didn't quite live up to its predecessor, it was still a massively enjoyable read and one I recommend.
The events of Latchkey take place about 3 years after the end of Archivist Wasp, which had finished with Wasp using the knowledge she's obtained to topple the post-apocalyptic regime in which she was living. She'd also had the offer to go with the ghosts she befriended in that book but had turned it down, then used her knowledge of the way they worked to keep herself isolated even in the midst of busy rebuilding. Wasp is now, to the people of the settlement where she lives, Isabel - living a fairly stable life until their peace is threatened by raiders.
Forced to access the tunnels under their settlement, Isabel discovers that there are more ghosts left over from the Latchkey project than just Foster and the ghost she'd initially befriended. In fact, in the intervening time, they'd been busy taking up Isabel's mantle and keeping those ghosts contained. So, alongside keeping the most vulnerable members of the settlement safe, Isabel joins forces with them again to try and find out just what is missing from both their memories.
To be perfectly honest, until Foster and the ghost turned up again, this book wasn't working quite so well for me as Archivist Wasp did. I think that book had such an unusual feel to it, with the cruelties of the system within which Wasp was trying to survive and the rules she was working hard to subvert, that the 'standard' post-apocalyptic feel of the first part of this one seemed a bit bland. On her own, or even with her fellow former-upstarts, Wasp just wasn't as interesting a character for me as she was when engaged with ghost-hunting. Still recommended though, with the caveat that you really need to read Archivist Wasp first!