This question has come up repeatedly this week, so I wanted to take a few minutes to address the topic in a short blog post today!
The simple answer is this: you can use whichever one works best for your book. You can even use all four in the same book if that’s what you prefer. There are no fixed rules here. However, you must label each one appropriately because they all serve distinct functions.
Author’s Note (nonfiction): A book's author writes the author’s note to explain why they chose to write about the topic in their book. This note gives readers a glimpse into what the author was thinking before, during, and after drafting the book. If a book is addressing a controversial topic, the author’s note will prepare readers for what’s to come and will explain to them why the author feels so strongly about their topic and how it personally affects them.
Introduction (nonfiction): An introduction introduces readers to the topic being addressed in an author's book. It can be written either by the author or by someone else and is used to explain why an author wrote their book, what qualifies them to write it, and/or why their story needs to be told.
Preface (nonfiction): A preface is like an author’s note in that it explains how the author came to draft their book. It provides the backstory and its relationship to the book’s topic. The preface is typically written by the author and usually tells a personal story. It's been stated that no one reads the preface, so it's best to skip it; I disagree. If well-written, with a captivating hook, the preface can set the tone for the rest of an author's book and add quality to its interest and flow.
Foreword (nonfiction): Someone besides the author almost always writes the foreword. This person is typically an expert in the topic being discussed in the book. Some authors even pick celebrities and well-known names to write their forewords. The point of a foreword is to lend credibility to the author and his/her book. Forewords don’t serve any other purpose besides being powerful advertisements for the release of the book.
If you choose to use one, two, three, four, or all four in your book, it is also important that you list them appropriately. The author’s note comes first, followed by the preface, introduction, and foreword. All of them come before the main content or first chapter of your book. In publishing, they are known as your book's front matter. Here's how to list them after your copyright page, dedication, and table of contents (if you have one):
I hope this will help you decide what to include in your book's front matter. I also hope that it will guide you on how to properly label each section. A professional editor can work with you if you're still unsure.