The Rhesus Chart
Bob Howard discovers that Vampires are a real thing through data analysis. Identifying an active cell he infiltrates a bank only to discover that leader of the cell is his former girlfriend.
Forced to bring her and her colleagues into the Laundry fold he gets caught in the middle of a tactical battle between two old and powerful vampires.
I was working my way through the Laundry Files as audiobooks and reached this one and had to switch to read the ebook instead. The banking vampires were just too annoying to listen to!
This is a pretty good Laundry book and probably the last before the Stross’ taste for bleak shit gets too much for me.