Well, my basic understanding was:
Summary
No Longer Human is a 1948 Japanese novel. It is considered Dazai’s masterpiece and ranks as the second-best selling novel ever in Japan. The themes include suicide, social alienation, and depression.
Thus Spoke Zarathustra is a philosophical novel by German philosopher Friedrich Nietzsche. It looks pretty complicated but one of its themes is criticism of Christianity including the famous dictum “God is dead”.
So:
The Bible is a book about God,
No Longer Human is a book about pessimism,
Thus Spoke Zarathustra is a book concerning pessimism about God!