A preface or foreword — the opening section of a book or paper where the author or another writer explains background, intent, or acknowledgments.
本の序文を書いたのは著者の親友だ。
The book's preface was written by the author's close friend.
その小説の新版には著者の序文が追加された。
The new edition of that novel included a preface by the author.
序文を読んでから、本の雰囲気がよく分かった。
After reading the preface, I understood the tone of the book better.