A book foreword should identify its writer. If the writer is well-known, the byline of the foreword is sufficient to put the reader on notice that the book in his hands should be taken seriously. A gentle reminder as to the foreword writer's credentials is useful. For example, the first paragraph of a foreword could read: "During my eight years as secretary of state, I learned that the perception of military strength is critical when negotiating with hostile parties."
The foreword should establish why the topic of the book is important, especially if the topic is obscure. The reader needs to know that the subject is worthy of consideration and there is a basis for the author's point of view. The writer of the foreword should establish why the book is worth the reader's time.
The writer of the foreword should be able to put the author's point of view in the context of related ideas. By framing the book as part of a greater tradition, or conversely, as a novel approach to a subject, the writer helps the reader take a first step toward engaging with the subject. For example, if the book is about using scrap materials to make furniture, the writer of the foreword can discuss the environmental benefits of using recycled materials and how the ideas in the book provide a new slant on that practice.
The writer of the foreword should not write with the intent of selling the book. After all, the foreword most likely will be read by people who have already purchased the book. A hard sell detracts from the author's credibility. Instead of selling the book, the foreword writer should establish why the book is an important contribution in the field.