The Screaming Staircase by Jonathan Stroud

Richly detailed world-building with believable characters and a good mystery. I haven't read any of the Bartimaeus books, so I loved my first real introduction to Stroud. I've recommended it to kids who can handle scary, because some of the ghost scenes are truly creepy.

I don't know what took me so long to read this series! It is so good! I like the ambiguous time period and the way the rules of this world are laid out. I like the way Lockwood, George, and Lucy are developing their friendship and working relationships. They are making mistakes and also sticking up for each other. And the ghosts and the mystery! So good.
This might be a new favorite series!