do you remember this?
although this is not the book he mentioned, it's linked to the "event" filmed presumably after romance is a bonus book, it sure refers to it as it is about books and reading. yes he wrote about the title of the book and maybe he did because the book was being talked about due to that event, where he suggests to read a book he likes, around the time he wrote the message. the "똑땅하다" is korean slang and it means "i'm very upset", "i'm disappointed", but said in a baby talk way, in a funny way, childishly. as you said, he was joking around. what's on my mind is: could it be he wanted to say he felt a little disappointment maybe because this campaign or whatever it is isn't going as he imagined or hasn't gained as much attention as he would like because he is not here in a way? maybe he thinks he's losing recognition as time goes by and he's less and less a part of the celebrity world since he is away? could it be he is disappointed about the results? could it be it's the book that's making him upset/sad (i don't know what it is about)? am i just reading too much into his words? what i know is there's always some meaning behind his jokes, because he wants others to think it's not a big deal. let's share opinions please, don't make me feel weird.