Notebooks are scattered around Stray in The Slums area, and you'll know one is close by when you see the sign of The Outsiders. The symbol is two dots above a small, incomplete circle with a gap at the bottom.
Whenever you see that symbol, look around and you'll find a notebook.
You'll obtain Momo's Notebook as part of the story. Once you meet Momo, show them the first Memory, which is a picture of the beach, and they give it to you after your conversation.
Zbaltazar's Notebook can be found in a flat that is reached by climbing the highrise next to Guardian.
Climb past the robots that are throwing cans of paint, and continue to climb until you reach the large, neon Outsider sign. Pull the power supply off the ventilation system to the left of the sign, then climb up on the vent next to the sign and drop through the fan.
Once you are in the apartment, climb on top of the tall pile of boxes near the robot portraits on the wall, then jump off, knocking the boxes over. Nudge the boxes to knock them open and discover Zbaltazar's Notebook.
To find Doc's Notebook, you'll need to head to the library above The Slums. Climb up above the alley where robots are throwing paint cans, then continue towards the back, beyond the couch with the TV.
From there, jump across the gap to reach a small balcony with a blue, glowing Outsider sign, then squeeze through the boarded-up doorway.
Inside, you'll find tons of books. Head to the back of the room and turn right. Hop over the giant piles of books to find a side room, then turn right to find a note you can translate. Do so and you'll obtain the Keys.
Head back towards the middle of the library to find an aisle with several more tall piles of books. Hop on top, then hop off to cause them to
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