The Cornered note is an optional artifact behind a door near the sewer ambush site in The Suburbs. After the runner and clicker attack, climb the stairs, go through the door on the left and find it on a bench by the bodies in the corner.
A short, desperate message from Kyle, written while trapped with some of the children as infected hammer on the door.
Kyle believes everyone else is dead and hopes Ish can still reach them, but the note ends with him deciding to spare the children a worse fate if help never comes. The bodies in the room and a painted message on the floor show how it ended.
Antiparticles
Antiparticles, issue 11 of Savage Starlight, lies on the console in the bridge of the wrecked fishing boat on the beach at the start of The Suburbs.
Boat note
The Boat note is an optional artifact on the bridge of the beached fishing boat at the very start of The Suburbs. Climb onto the deck, step into the wheelhouse and grab it from the console beside the Antiparticles comic.
Bomb: Containment
Bomb: Containment is a training manual on a shelf in the sewers of The Suburbs. It makes nail bomb blasts 1.5 times wider.
Father's note
The Father's note is an optional artifact in the brick house on the second street of the Pittsburgh suburbs. Go up to the second floor, enter the left bedroom and pick it up from the dresser.