The book in episode 21

 MnS:

Does anyone know the title of the book Jung Hae In's character read?

When we fall in love, must we get into it knowing whether it will be fruitful or not? Can we approach love in that manner? We mustn’t. We cannot be calculating in any aspect, because, after all, we love who we love. When one falls in love, it just happens. That’s all there is to it. So let’s not wallow in despair, restrain our feelings, or smother the light…”

It's the book

"the complete letters of vincent van gogh"