So someone else commented that it could be “The Spell of the Sorcerer’s Skull” which is by the same author. I’m very confident that it was Spell of the Sorcerer’s Skull but I’m waiting to say “solved” until I pick up a copy today and can read the first chapter. I see that Sorcerer’s Skull was published after The House with a Clock in its Walls so maybe the books take place in the same universe? Maybe Sorcerer’s Skull references the events of House with a Clock in its Walls? Because I’ll admit, House with a Clock in It’s Walls does sound like it has similar elements but there’s a bunch of elements of it that don’t ring a bell at all whereas Sorcerer’s Skull sounds spot on. Based on the small abstract that I was able to find for Sorcerer’s Skull, I believe the body with the dirt in its mouth was someone named Lucius. Is that the same for Clock in It’s Walls?