Oh dang, wasn't expecting that. Guess she got the same book, eh?
Or it's the same picture book. We've only heard about the picture book before now or seen it in flashbacks- we never saw it in the present before, far as I'm aware. It either was always Kosaki's, or it was someone else's (likely Chitoge's, in that case, unless another girl is introduced with a connection to that book), and Kosaki ended up keeping it after everyone parted. Far as I'm aware, we haven't seen it in Chitoge's room, have we? Just that diary of her's. Maybe I'm misremembering, though, but that's my impression. (edit: also, this is assuming the line about 'Chitoge's picture book' was just Hana assuming it was her daughter's because she was in possession of it at the time of her visit that year, at least in the case that it wasn't Chitoge's originally)
And I'll just say... Hana really sees through that boy. She can tell he cares a lot for Chitoge. What else is someone supposed to think when he would put in so many days in a row of corporate work to free up time in her mother's schedule, then start racing around town to find and then take her to her mother personally at all costs? Then do everything he could to clean up the mess said mother's delay in schedule caused?
He's just still in denial that he cares about her that much because he's still clinging to his crush on Kosaki. But when has he ever put in the effort he has for Chitoge's sake over the time he's known her into doing things for Kosaki? And a number of the times he's gone off for Chitoge's sake has left Kosaki in the lurch, as well. The one time, so far, that he's helped Kosaki with her family's sweet shop doesn't begin to come close to the work on behalf of Chitoge, in my mind.
Maybe there's more things for Kosaki, and the others, in time, but I feel like he's going to continue having to run off and 'save' Chitoge, so to speak, many more times during this story.