Dear Geist,
What exactly is a preface? How long should it be? My editor has asked me to write one for my first book of non-fiction—a collection of essays, coming out in a few months—and I’m too embarrassed to admit my ignorance. Is it like an introduction?

Dear Learning,
A preface, almost always written by the author, is a short piece at the front of the book that speaks directly to readers about the book: what inspired it, how your research and/or writing were carried out (if unusual), special challenges—whatever context stands out for you as you greet the reader. Acknowledgements can be incorporated in the preface, or they can comprise a separate section, usually at the back of the book. Other written “front matter,” as it’s called, may include a foreword—a short, enthusiastic endorsement of the book, usually written by someone other than the author; and an introduction—info for the reader on approaching the book, sometimes including chapter summaries, also written by someone other than the author. Not every book has all of these elements; some have more. If you page through your own essay collections, or browse them at your library, you’ll get a good sense of the tone and purpose of each. Feel free to ask your editor for advice on subjects or ideas to focus on, as well as an approximate word count for the preface.
—The Editors