Sheri Rosen raises a key question. She asks "how much of an 'expert' must a nonfiction writer be for agents to be interested in representing him?" While each agent spells out specific submission requirements (and writers will save valuable time and effort to check these out), most have the expectation that an author must be qualified in the subject matter of the book. It is therefore essential that one's qualifications and expertise be stated in the query letter. For example, an author who writes a memoir of personal child abuse, is not an expert in child psychology, and is no authority in that field. Likewise, an author who writes about conquering obesity, may offer a compelling memoir about his/her success with dieting, but is in no position to write an authoritative exposition on any aspects of nutrition.
While life experience is essential in writing both fiction and nonfiction, books on self-help, health and nutrition, medical procedures, investing and finance, politics and current events, etc., require expertise in the area as well as specialized formal training and often university degrees.
As I stated, agents have specific submission requirements. Some accept query letters accompanied by a formal proposal. Agents like myself accept query letters only, so it is imperative to state in one or two pages the concept of the book and indicate the qualifications of the author. I'm especially sensitive to highly qualified nonfiction writers, as my overall goal as an agent is to bring important works to press. It therefore behooves the writer to let me know why he/she is competent to write about the proposed subject.
Given the fact that my agency receives about 7,500 queries annually, and that we offer representation to only a few projects at a time, many well written letters are rejected due to a multiplicity of reasons. Therefore, the diligent writer needs to persist in searching out the right agent for representation.
In addition to the author's qualifications, is the all important platform. But that's another subject.