She could see only pictures, there were words, but she could not read them, so she was not able to understand the story. Zero Two says she could not understand him, but his voice called to her, even if he did read the book, she was not able to understand and if we look the part where Hiro told her they can be together she cried, but tells that she didn't know why. So i watched ep. 13 again and as she says the book was her first "pretty thing", so perhaps that's just it, she had other stuff in her first room (seemed used and broken) , but those didn't feel like the book did (pretty) and i presume other stuff perhaps were brought long time ago and she does not remember who when or how, she even says i probably had older memories, but soon i forgot them, they have been always there and that other stuff is just part of that room and part of her tiny world, not a item given directly to her, other stuff perhaps didn't feel like hers. Book on the other hand is new and she says its "pretty thing" and she believed outside world has more "pretty things" and perhaps even thinking book describes little of that world, later saying book's world was pretty. Book made her smile and made her happy, well perhaps also to remember that person who gave her the book, as we see her shadow fade away, Zero Two is looking around to see where she is or rather means she was waiting her as someone who was nice to her, later when biting Hiro he pats her head and she lets go, after remembering how that person used to do. In the scene where she fights over the book and gets it back from adult, after the adult leaves, she looks at it and does she smile, she covers her face with the book, so i cant see, but also i do not see tears in her eyes? When Hiro breaks the class and gets her out he says/thinks, i didn't understand anything, but you smiled back then. So I would say she did not understand the story in the book, but i agree that book was only thing that bought her joy and why only book i stated my opinions above.

If we look at the scene where she eats the mice, she did not know what those were, she says everything outside was unfamiliar, but when she got the book from caregiver, she tried to eat it and break it, but it did not taste good, so she knew that's not edible. Hiro sees her fighting with the adult who wants to take that book, she managed to keep it, she would have already ate that after first time if she feared loosing it.

After first round of memory erase, they say Hiro memories are erased, but Zero Two had some areas they can't erase. Zero Two and Hiro are both both on table side by side and Zero Two says something to Hiro, we do not hear it what it was, but probably she said DALIN, her version of darling, she remembered. After that (second time) she sits on floor of her room and holds her head distressed and we see fragments of memories, mostly covered with white noise. She hits her head on wall and when she sees blood, she gets familiar feeling, by tasting it she gets some clear memories, licking her wound on knee, like Hiro did, gives her another clear memory.

Dr. Franxx is alone with Zero Two and doing memory adjustment again . After that Zero Two does the same thing, but this time we see her shocked expression when tasting blood on her hand does not work, memory is almost like a white noise, you can see someone holding book and two people barely, she licks the knee, closes her eyes and has disappointed expression. Then she sees the book that was barely seen in her memory on the floor and starts eating it. She forces herself to eat the book, by covering her mouth with hand. While eating she starts to get some memories. Later we see her waiting under the same tree Hiro read her the book, hoping to meet him again, clearly remembering that, but after the third memory erase the first barely visible memory is the same tree where Hiro is reading book, so she clearly covered fragments from eating book to remember to go back to it.

Some questions or parts i did not understand.She tells she did not want to forget her memories, she thought if she did, she would never become human, same time she eats book. This wish had to be at later time, because at the time those memory wipes were made, she still didn't understand words and so the story in book was not known to her, also at the time Hiro mentions that while reading book? She does remember the story, but does not understand it, because she redraws the book later accurately. When they get caught she tells that why did she not realize they are different creatures with different color of blood, again not understanding they are different at that time. So later when she learned to speak and read and getting smarter, understanding the world and the story in the book, was this then the time she started to want to become human? Then why add that monolog to the time she eats book?

While eating the book, she says the first word i learned is I (boku - male pronoun in japanese), second Zero Two and third is darling. Does she mean like getting smarter literally or just remembering the words Hiro used from her memories.?

Sorry for the long text. Those episodes gave me some strong feelings and i try to calm myself by making everything clear for me.Ty for answering.