[Maybe she ultimately shouldn't actually be glad that more of their companions are getting trapped here but when Yona sees the video of Jaeha, it feels almost like she can't help but be both surprised and glad for it.]
Don't leave the book behind, it's all right-- they're for us to communicate. Can you see me? [Using the video function hoping the response will reach him, she's already on her way out to try to meet up with him.]