Die fröhliche Wissenschaft
("la gaya scienza")

Friedrich Nietzsche referred to it as the most personal of all his books. In The Gay Science, Nietzsche for the first time proclaimed the death of God and outlined his doctrine of eternal recurrence.

Any detailed explanation I could attempt here would probably not do this book justice, but the summary provided by renowned Nietzsche scholar Walter Kaufmann in the preface to his translation is perfect:

"This book is a microcosm in which we find almost all of Nietzsche: epigrams and songs, aphorisms and sustained discussions of philosophical problems, ethics and theory of knowledge, reflections on art and on the death of God, the eternal recurrence and even Zarathustra. The abundant material Nietzsche added to the second edition, translated here, makes plain that he did not consider this book dated by Zarathustra and Beyond Good and Evil. It is not merely one corner of a large edifice or one fraction of a system. It mirrors all of Nietzsche's thought and could be related in hundreds of ways to his other books, his notes, and his letters. And yet it is complete in itself. For it is a work of art."

One of my favourite quotes from The Gay Science:

"... What is new, however, is always evil, being that which wants to conquer and overthrow the old boundary markers and the old pieties; and only what is old is good. The good men are in all ages those who dig the old thoughts, digging deep and getting them to bear fruit--the farmers of the spirit. But eventually all land is exploited, and the ploughshare of evil must come again and again."
(I, iv)