Originally the term masterpiece referred specifically to a piece of work by an apprentice or journeyman who wanted to become a master craftsman in the European guild system of the Middle Ages. That piece was often displayed in the guild house as evidence that the worker had sufficient skill and expertise to warrant being a master at his chosen craft.
Every Recognized Expert should have an Expert Proof Product (EPP), or better yet – several that make up their Expert Proof Materials (EPM). An EPP could be a book, or a DVD or an audio CD, almost anything that demonstrates that the person is an expert in a particular subject, has definite ideas and opinions, and can reasonably be considered an expert on their subject.
In creating a non-fiction book (a novel could be considered EPP if the person is establishing their credibility as a fiction writer, or perhaps as a creative writing teacher), you don’t have to start from scratch. Putting together a series of articles or blog posts into a narrative that explains or explores the Expert’s niche is perfectly acceptable. Adding in some charts, perhaps even a transcript of an interview or two is also a possibility.