The preface can state the author’s overall experience or his professional qualifications to write about a topic. He can indicate his experience on a topic and his prior written works. The preface should be written in first person in a reader-friendly manner. The author should use his own writing style and voice to address his audience.
In the preface, the author should indicate clearly why he wrote the book and what the book endeavors to accomplish. He should mention why he feels the topic addressed is important. He can choose to acknowledge the person who inspired him to write the book, or he can mention any incident that spurred him to take up this writing task.
A potential reader who reads the preface will be interested in finding out how the book can benefit him. The preface should precisely state that. The author should attempt to put himself in the shoes of the reader and mention the things that a reader might want to know. After reading the preface, the reader should be convinced that he would enjoy the book.
A brief explanation on the book's structure and how it should be read can be included in the preface. The author can mention the distinctive features of the book and what makes it unique. If it's an academic book, the author can mention what courses the book addresses and which discipline it caters to. Any special notes relating to the structure can be included, too.