The House of Dies Drear has what's commonly known as a happy ending, for the most part. After the Skinners, the Smalls, and Pesty frighten the Darrows out of their wits, a couple of important things happen. The Darrows have been stopped from antagonizing the Skinners and the Smalls, and Mayhew and Mr. Pluto have come a long way toward mending their relationship. Plus, Mr. Small has indefinitely postponed telling the foundation about the treasure of Dies Drear, which means that Mr. Pluto can continue to live life the way he wants to.
Mr. Small definitely does this out of kindness and respect for Mr. Pluto, but he gets something out of it, too. Mr. Small is all about history – teaching it, learning it, and living it. Dies Drear's treasure trove of precious antiques appeals to him for some of the same reasons it appeals to Mr. Pluto, because it's history and it helps tell the story of how we came to be where we are now – not because it's worth money. So, cataloguing the treasure is more than an excuse to stall telling the foundation about the treasure. It's a chance for Mr. Small to live his dream of being immersed in history, history he can touch and feel.
For Thomas and the rest of his family the ending means freedom from fear. They can now enjoy the house and the town without worrying about people breaking in while they sleep, or while they are gone. It means they can trust Mr. Pluto and Mayhew, and so feel welcome where they live. But, there are a few loose ends that just might be covered in the sequel, The Mystery of Drear House, the second and final volume of The Dies Drear Chronicles.
First, there's the issue of ghosts. Is the house haunted or not? Mr. Small would say, "No!" But some of the other characters might not agree with him. By the end of the story, we know that the scary things that happen in the house are the work of the living, not the dead, or the undead. But, does that mean the house isn't haunted? Not according to Mr. Pluto. When he and the others are waiting for the Darrows to arrive so they can scare him, we learn that he's…
frightened that he might see the real ghosts of old Mr. Drear and the two slaves, as he had seen them before when he was sick and tired with despair. […] [The Darrows] had followed him down the years, as had those ghosts, so that, oftentimes, he couldn't tell which he was seeing. (18.43)
Thomas probably believes in ghosts, too. Ultimately, the story leaves open the possibility that ghosts do exist. It also leaves us not knowing if the town will accept the Smalls, or whether things will get even worse now that the Darrows have been offended. From what we saw in the church, most of the town has sided with the Darrows and not with Mr. Pluto.
For things to be completely good, the Skinners and the Smalls will need to heal their relationships with the Darrows. After all, River Swift Darrow (the grandfather) and Mr. Pluto used to be friends. River Swift is dead now, but his children and grandchildren aren't. Maybe a friendship between Mac Darrow and Thomas can form a bridge between the three families.