The Notebook Light Finds Belongs to Sidoh, Not Ryuk
SPOILER!
Stolen by Ryuk · Dropped for a Human to Find
The first Death Note Light picks up is not Ryuk's personal notebook. Ryuk stole it from another Shinigami, Sidoh, then dropped it into the human world because he was bored. Ryuk remains beside Light because he is the Shinigami attached to the notebook in human hands, but the physical book itself began as Sidoh's.
Why it matters
The story's first notebook already has a history Light does not know. Years later, its original owner comes looking for it, turning an object Light treats as his weapon into someone else's missing property.
Sidoh: He Wants His Notebook Back, Not Kira's War
SPOILER!
Original Owner of Light's First Notebook
Sidoh is almost comically disconnected from the human struggle built around his notebook. Ryuk stole it. Light built Kira with it. The task force, Higuchi and Mello all handle it. Sidoh eventually enters the story for a much simpler reason: he wants his property returned.
Why it matters
Sidoh is useful precisely because he has no investment in Light's ideology. His arrival reminds everyone that the Death Notes have owners and histories outside the human plans built around them.
Gelus's Notebook Reaches Misa Through Rem
Gelus Dies Saving Misa · Rem Delivers What He Leaves Behind
Gelus dies after using his Death Note to
save Misa's life. Rem witnesses his death, takes the notebook he leaves behind and later gives that notebook to Misa. Misa therefore enters the story with a Death Note whose previous owner died specifically because he cared about her.
Why it matters
Misa's notebook is tied to protection before she ever writes in it. That history follows her into Rem's decisions and eventually into the notebook's later owners.