Maybe the answer to the riddle is linked to the question marks on the title page sundial.

Have you noticed the sundials on my notes page and the book page?

Overlay my notes page carefully on top of the title page sundial. When aligned perfectly, certain letters will appear on both items. Match them precisely to reveal the missing letters of the sundial, then use them to decipher the code on the postcard.

2 Word Phrase:

1) Colour of the sky.

2) Entrance to a home.

The suspect was seen at midnight.

A torn letter was found in the garden.

The clock stopped at 11:47.

I heard footsteps in the hallway.

The door was left ajar.

A shadow moved past the window.