The non-fiction genre is the large umbrella under which all of those books that are not fiction fall into, such as memoir, history, biography, sports books, politics and cookbooks. Although they are not inventions of the author, they can seem as different as night and day. The manuscript lengths vary, but most are between 60-80K words. The target audiences are as different as the books themselves.
Some of the most commercially popular sub-genres in the non-fiction genre are the memoir and biography categories, though the target audience seems to depend more on the topic than the audience. Jaycee Dugard’s autobiography, A Stolen Life, is likely to appeal more to women than men for its subject matter of a child kidnapped at age 11 and held captive for 18 years. Jane Leavy’s book on baseball legend Sandy Koufax: A Lefty’s Legacy, however, will probably appeal more to men. Not that women and men can’t enjoy both; we know they do.
One of the most successful non-fiction authors of recent years, Erik Larson, crossed both sub-genre and demographic lines with Devil in the White City. The main reason this book was so successful wasn’t necessarily because of the specific characters. It was successful because it read like a narrative, and it just so happened that everything in the book was true. History writer David McCullough (1776) produces massive histories that are compelling as well as factual. The best non-fiction books are those that make readers forget they are reading a factual account, leaving them hungry to delve deeper into a given topic.
If you write non-fiction you might consider exploring the American Society of Journalists & Authors (ASJA), a professional association of freelance/independent journalists and nonfiction book writers. It shares information about markets, writing rates, contracts, editors, agents, and more. Additionally, this website for Writers and Editors is a great resource for non-fiction writers, as are the numerous conferences for non-fiction writers.