Ilse's Notebook is a side story placed before chapter 19 in the fifth volume of the manga Shingeki no Kyojin.
Summary
The lone survivor of a Survey Corps expedition, Ilse Langner, is running through the forest after her squad was killed and their horses dispersed by Titans. Making matters worse, her 3D Maneuver Gear was irreparably damaged, leaving her defenseless. She writes down all that has happened in a small notebook as she tries to make her way back to the wall gates.
Coming to a heavily wooded area, a six meter class Titan suddenly appears and backs her up against a tree. Scared, she continues to write in her notebook and wishes she could have been better to her parents. Inexplicably, the Titan starts to murmur distinct words, specifically, "A subject of Ymir", "Lady Ymir" and "well met" before bowing seemingly in respect. Surprised, she writes down her experience and tries to communicate with the Titan. At first she asks what it is and then where it is from, but doesn't receive an answer. She then asks why the Titans eat people and is again met with nothing but groans.
Frustrated, she asks why they eat when they have no need for nourishment and calls Titans worthless lumps of flesh. The Titan reacts by grabbing the skin on its face and drawing blood while giving Ilse a cold stare. She tries to run away, but the Titan soon overtakes her, picking her up and biting her head off with a look of sorrow on its face. The notebook she was writing in is left sitting in the grass, only to be found a year later by Levi and Hange.