Confessions (400 AD) by Saint Augustine of Hippo.
In order to avoid the wrath of the moderators due to the obvious religious and political content of this book, I’ll go for a brief overview.
Confessions is without any doubt one of the most important books in Western history. It has shaped theological and philosophical thought and its legacy lives on. Composed by 13 books, Confessions is substantially an autobiography. However, contrary to other biographies, while there is a certain degree of chronological or factual description, the text is mostly focused on the pondering about the human soul or, if you will, the human condition. Starting from his youth, the book traverses several stage in Augustine’s life.
One of the most important areas is Augustine’s search for the understanding good and evil, which leads to his famous evaluation (won’t discuss it here to avoid PRSI content).
The book is not really easy, but it’s very readable with a modern translation and good footnotes.
I recommend listening to this non-religious, great introductory lecture by Dr. Sugrue: