Hilda's sketch book is the book that Hilda always carried with her while still living in the Wilderness, using it to make sketches of any creature she came across.
Early in Chapter 1: The Hidden People, the book got eaten by a Troll, but the creature returned it to Hilda after Hilda removed a bell that was tied to the Troll's nose. This same scenario also happened in the first graphic novel, "Hildafolk", and in the novelization "Hilda and the Hidden People". In that book, the adventure later proved useful to Hilda, since she could use the troll saliva stuck to her sketchbook to get into the Caves of Kismet.
In Chapter 2: The Midnight Giant, after Jorgen accidentally destroyed Hilda's house, Wood Man found the sketch book between the rubble and returned it to Hilda, remarking he had no use for it himself and it smelled like troll spit.