There isn't one. Nonfiction is very genre-specific -- the small to midsized publishers that are your best bet as a beginning writer specialize in one kind of nonfiction or another. For example, most presses that do occult nonfiction don't do anything else, or do at most some closely related field such as alternative health. Your best bet is to look on your bookshelves and online, see what publishers issue books in the fields you want to write in, and go to their websites to see what their submission requirements are.