Who Knew There’d Be Ghosts? By Bill Britain, 1985.

Tommy Donahue and his friends, Wendy “Books” Scofield (the smartest kid in their class as well as being pretty tough) and Harry “the Blimp” Troy (known for being the tallest and biggest kid in their class), prefer playing around the abandoned Parnell house instead of at the park because they like to play games of pretend, based on adventure stories that Tommy has read.  It’s hard to play games of pretend in such a public place as the park because other people either laugh or think that they’re just getting in the way.  Almost nobody goes near the old Parnell house because people think that it’s haunted.  They’re right; it is haunted.

Some people in their town have been trying to arrange for the Parnell house to be turned into a museum because the Parnells were the founding family of their town, but the movement hasn’t been able to raise the money needed to renovate the place.  Now, Tommy’s father, a lawyer, has been recruited to arrange for the house to be purchased by a private citizen who says that he wants to renovate the house and use it as his own residence.  However, Tommy and his friends overhear the buyer, Avery Katkus, and a confederate talking as they look over the house.  Mr. Katkus isn’t interested in the house at all; he wants something valuable that is hidden inside.  When they hear the two men plotting to sneak into the house at night to do some searching for this mysterious something, the kids decide that they will come back at night and watch for them to find out what they’re looking for.  The kids don’t want anything bad to happen to the house because they’ll lose their private playground.

Tommy is the first to go and check out the Parnell house at night, and that’s when he meets the ghosts, Horace and Essie Parnell.  At first, Horace tries to scare Tommy away, but when Tommy explains that he only came to keep watch, Horace asks him what he means by that.  Tommy explains to him about Mr. Katkus, and Horace says that he could use Tommy’s help.  Years ago, Horace’s father made a dying wish that all members of their family should be buried in the family cemetery on the property of the house.  Most of the members of the family are buried there, but Horace, who was killed during the Revolutionary War, and Essie, who accidentally fell overboard from a riverboat and was permanently lost in the Mississippi River, were only two Parnells who were not buried on the property, so their spirits are now bound to the house.  Naturally, Horace and Essie are concerned with the future of the house.

Tommy tries to tell his friends about the ghosts, but they don’t believe him until they see the ghosts for themselves.  When the three kids return to the house the next night, Horace saves them from being attacked by Mr. Katkus’s hired confederate.  Now convinced of the ghosts’ existence, Harry and Books are eager to help save the house, and the key in doing so is discovering what kind of hidden treasure the house holds.

The book is available to borrow for free online through Internet Archive.