Lucy and Oliver don't actually hang out with each other at the beginning of the book, which I thought was especially realistic. Oliver is busy with Teddy and helping his father, and Lucy is more interested in investigating the animals and the original owners. They both learn a lot about the house, but it doesn't make much sense until the communicate and share what they know. It's a good thing they do, because The Gar and his minions are NOT fooling around and want to take over the house. If the children weren't there to stop them, who knows how widely the evil would spread!
This is slightly reminiscent (how could it not be?) of the newly popular Bellairs' The House with a Clock in Its Walls, and will be popular with readers who like spooky tales like Oh's Spirit Hunters, Currie's The Peculiar Incident on Shady Street or Schwab's City of Ghosts.