The Hollow Boy, by Jonathan Stroud
I just love the Lockwood & Co. series--I think it's really fun. This is the third book.
Setup: In an alternate London, it's been 50 years since the Problem started. Ghosts are real, and they can kill you in a variety of unpleasant ways. They are also invisible to adults; only talented children under 18 or so can sense them, so ghost-hunting teams are young. Lucy, together with the dashing Lockwood and the brilliantly analytical George, makes her living hunting Visitors. There's a slightly steampunk feel to the setting and it's a bit surprising to realize that this is a modern London, not a Victorian one.
In this third book, Lockwood and Co. are getting to be known, but are still unable to compete with the larger firms. They haven't even been invited to help fight the sudden massive surge of dangerous Visitor activity in Chelsea, though of course they tackle it anyway, in their own way. Lockwood also brings in a fourth for the team--the polished and efficient Holly, whom Lucy immediately hates. Lucy's feelings of inferiority, and her crush on Lockwood, form a secondary plot strand that adds emotional depth to a fast-paced horror adventure.
Great fun reading for 10+ ....those who aren't prone to nightmares, anyway.