The hardest book I have ever written

Mr. Murakami:
I read one of your books a few years ago, and I haven't stopped since. I love the way you write, and how you surprise me in every page.
I have enjoyed every single one of your books and I want more, I have a few favourites, but I wanted to know yours.
Was there any book that you enjoyed writing the most?  Or a book that you found hard to write? 
If you had the oportunity that I'm having right now to "talk" in some way with any author that you enjoyed reading, who would it be?  and what would you ask? 
Thank you for doing this, I know readers appreciate it.

A die-hard fan from Chile.
(Crazy Cat Lady、女性、20歳、Student)

Dear Crazy Cat Lady,

Thank you for reading my books.

I don't think I have a particular "favorite" book. I am always thinking of the book I am currently writing, or the next book I am about to write. I, as an author, am not interested in the past.

The hardest book I have ever written so far must be THE END OF THE WORLD AND THE HARD-BOILED WONDERLAND, because I could not decide how to end the story. I had various conclusions, and all of them seemed reasonable in each own way. It was truly an ambitious book. The novel might have been a little too ambitious for me, I was a young writer then.