Do you have to be an expert on something to get a nonfiction book published?
Usually some level of expertise is necessary to produce a successful nonfiction book, especially for fields such as health, self-help, or parenting, where no one will trust your advice without recognized credentials. Your background must instill trust in the readership. (Would you, as a reader, trust a healthcare book by an author with no medical degrees or experience in the healthcare field?) However, some types of nonfiction, especially biographies, historical surveys, and creative nonfiction/memoir, can be written by anyone with good research skills plus organizational and writing abilities.