Write about why you wrote the book. If the book is a work of fiction, write about how you got the idea for the story.
Give a brief description of the book, but do not give substantial details. Give the reader a very brief preview of what she will find if she keeps turning the pages. Explain the topic's importance.
Tell your readers how you did your research for the book. For instance, your research may have involved countless hours at the library, interviews with professionals or a trip to another country. Briefly relate your research adventures.
List acknowledgments. Briefly take the time to thank those who helped you along your writing journey. Obviously, you'll never be able to include every person or institution, so try to keep this as brief as possible without hurting anyone's feelings.
Recount any troubles you encountered along the way. The journey to write a completed book is strewn with triumphs, failures and near misses. Tell about those adventures in your preface.
Acknowledge your intended audience and why you think the topic or story would be of interest to them.