It seems to me that there are three, perhaps four, types of introduction for a book:
1. The arresting anecdote. This introduction revolves around with some kind of anecdote or story that in some way encapsulates the themes or concerns that are to be explored in the pages that follow. It might begin: “The morning of October 10, 1492, dawned bright and clear…”
2. The straight talk. This introduction outlines the argument of the book. It is essentially the book crammed into twenty pages, rather than two or three hundred. It might begin: “This book is about…”
3. The self-reflective gaze. This introduction discusses the book, and perhaps its style, as a way to explain the processes that led up to its writing, or to forestall criticism. It might begin: “I first had the idea to write about…” Or maybe: “Some will say that the time for a book of this nature is long past…”
4. The missed opportunity. This introduction is less about the book itself, than it is a separate essay about the topic that the author now realizes he or she would have wanted to write about. It might begin any old way.
NB in all four cases, the last three or four pages of the introduction are probably taken up with a chapter-by-chapter outline (a paragraph for each chapter) of what is to come.