The Creeping Shadow (Lockwood & Co. #4), by Jonathan Stroud
I really enjoy these Lockwood & Co. books. They are just so much fun, and they keep moving; the series isn't getting stale.
In this fourth installment, Lucy has been on her own for some months; she's lonely, but she doesn't have time to do anything but work and sleep. Lockwood offers her a commission for a particularly tricky job, and then a little kid shows up asking for help for his village, which is coming under massive attack. What is causing this sudden and deadly outbreak in a remote village?
In every book, we (and the characters) find out a little more about the world and how the Problem works. They tackle different things every time and find out more.
Lucy, the narrator, is a great character; she's smart and practical, and very matter-of-fact about her work. She's never clearly described, but she's messy and bedraggled and impatient with anyone who isn't fine with that. She doesn't have time for it, and she's no good at it anyway. Still, it makes her nervous, which makes her angry, and she works through her difficulties without ever discussing them outright.
Fun middle-grade ghost stories. I love 'em.