Nonfiction submissions bear little similarity to fiction submissions. Fiction editors focus on plot, writing, and mass market appeal, which means they will want to see sample chapters and a synopsis. Nonfiction editors require a full proposal, which means you will have to submit detailed chapter descriptions, explain how your book will fit into the market, and establish your expertise. Even if your book is finished, a proposal is a requirement. (The sole exception is memoir, which is similar to fiction.)
Make sure to read the submission requirements of these publishers very carefully, and adhere to them. Don't give the publisher an excuse to throw out your proposal!
Note: For a list of hundreds of publishers accepting unagented books (by genre) see: Publishers Accepting Unagented Manuscripts
Adams Media publishes a broad range of nonfiction topics including, business and careers, parenting, pets, personal finance, self-help, travel, weddings, writing. In 2003 Adams Media was purchased by F+W, A Content + eCommerce Company one of the largest special interest publishers in the world. Read their submission guidelines here.
Allworth Press publishes business and self-help information for the general public and creative professionals. Read their submission guidelines here. Seeking: Graphic Design, Business, Performing Arts, Interior Design, Art, Theater, Web Design, Book Arts, Photography, Crafts.
Andrews McMeel Publishing is the country's premier calendar publisher. Its core publication categories include: comics and humor, puzzles and games, inspiration and gift, and comics for middle grade children. Interestingly, they also publish poetry.
Career Press, Inc. publishes quality, nonfiction books for adult readers seeking practical information to improve themselves in careers, college, finance, parenting, retirement, spirituality, and other related topics. Read submission policy. Seeking: Business, Career, Job Search, HR & Work Place Issues, College Preparation, Small Business/Entrepreneurship, Motivation/Self-Help, Management, Marketing/Sales, Negotiation, Study Aids, and more.
Free Spirit Publishing. "Our mission is to provide children and teens with the tools they need to overcome challenges and make a difference in the world. We publish high-quality nonfiction books and learning materials for children and teens, parents, educators, counselors, and others who live and work with young people."
Gryphon House, Inc. publishes books that help teachers and parents enrich the lives of children from birth through age eight. They do not accept children's books. Seeking: Educational resource books for parents and teachers aimed at young children. Read submission policy.
Hohm Press is committed to publishing books that provide readers with alternatives to the materialistic values of the current culture and promote self-awareness, the recognition of interdependence and compassion. Their subject areas include religious studies, natural health, parenting, women’s issues, the arts and poetry. Read submission guidelines here.
Menasha Ridge Press and its sister company, Wilderness Press, publish primarily books on adventure, nature, and wilderness sports activities (excluding hunting and fishing). Read submission policy.
OneWorld Publications was founded in 1986 by husband and wife team Juliet Mabey and Novin Doostdar as an independent publishing house focusing on stimulating non-fiction. Located in the UK. Read guidelines here.
Osprey is a UK publisher specializing in military history. Read submission guidelines here. (Scroll down to "Proposals.")
Skyhorse publishes a broad range of nonfiction titles. They only accept submissions that fall into their categories, so make sure you consult their list before submitting. See submission requirements here.
The New Press is a non-profit independent publisher focusing on contemporary social issues, with an emphasis on race relations, women's issues, immigration, human rights, labor and popular economics, and the media; education reform and alternative teaching materials; cultural criticisms; art and art education; international literature; and law and legal studies. The Press has also taken a leading role in publishing a wide range of new work in African American, Asian American, Latino, gay and lesbian, and Native American studies, as well as work by and about other minority groups. They are very selective.
Ulysses Press is a small house focusing on niche markets. Genres they publish include Cleanse, Arts and Crafts, Coloring, Gifty Grabs, Fashion and Beauty, Healthy Eating, Krav Maga, Mmm Bites, Pop Culture, Prepping (for the apocalypse), Fitness, Special Diets, Trivia, Working Out, Rehab and Injury Prevention.