The ending of The Monstrumologist is gloriously open to interpretation. The author who has published Will Henry's journals (a.k.a. not Yancey) has gone out to his grave—ostensibly to pay his respects—and to further search for evidence that any of his stories could be true. Although he's trying desperately to talk himself out of believing anything Will Henry wrote, a small part of his subconscious continues to assert that it could have happened.
So on a whim, he scratches the dirt above Will Henry's grave and a tiny, wormlike creature that fits the description of B. arawakus crawls out of the soil. He contemplates it for a moment and then hightails it outta there:
Get real, I told myself, trying to laugh it off, and that brought to mind something else Will Henry had written. The words followed me as I beat a hasty retreat to the car and, beyond that, to my modern life in a world where room for monsters shrinks by the hour.
Yes, my dear child, monsters are real. I happen to have one hanging in my basement. (Epilogue.39-40)
Will Henry's creatures have definitely taken hold in the poor guy's brains, that's for sure. And as for us, the intrepid readers, we are left wondering whether it was just an ordinary worm… or not. Could it have been a B. arawakus, crawling through the soil looking for a new host after being released by Will Henry's rotting corpse? Only time will tell… or the next book in the series.