"She's weaponised it. She's weaponised her library. This is where the Time-Turner will be. Solve the riddle and we'll find it."
-- Delphi (CC1.19)
When Scorpius Malfoy, Albus Potter and Delphi illegally enter the Ministry of Magic and sneak into Hermione's office to find the Time Turner, they are surprised at the number of dangerous books contained in the her bookcase (CC1.19).
Opening up "My Eyes and How to See Past Them" by Sybill Trelawney, the first of a series of three riddles is uncovered:
The first is the fourth, a disappointing mark
You’ll find it in parked but not in park....
The second is the less fair of those that walk on two legs.
Grubby, hairy a disease of the egg.
And the third is both a mountain to climb and a route to take....
A turn in the city, a glide through a lake.
They have to answer the riddles under pressure while the bookcase - Hermione's "Weaponised Library" - is simultaneously trying to pull them inside (CC1.19).
The second riddle is spoken by the book "Dominating Dementors: A True History of Azkaban":
I was born in a cage
But smashed it with rage
The Gaunt inside me
Riddled me free
Of that which would stop me to be.
Although Scorpius is in a panic with Albus and Delphi trapped in the bookcase, he solves the third riddle from the book "Marvolo: The Truth":
I am the creature you have not seen
I am you. I am me. The echo unforeseen.
Sometimes in front, sometimes behind,
A constant companion, for we are entwined.
They are then able to extract the Time Turner from the "Shadows and Spirits" book and continue on their mission (CC1.19).
All three of the riddle answers are related to Dark Magic: Dementors, Voldemort and shadow.
When Hermione solved the Potions riddle she said that "A lot of the greatest wizards haven’t got an ounce of logic, they’d be stuck in here for ever" (PS16). Scorpius and Hermione think in the same way ("Blimey! There are two of them!", CC4.10), which feels like why Scorpius is the one to solve the final Book riddle.