Feather Schwartz Foster has become one of the foremost authorities on the First Ladies of the White House. She writes about them, gives presentations and teaches workshops. Feather through her research and writing, Feather has become a specialist or authority so her entertaining programs about the First Ladies are popular.
Her book, Ladies: A Conjecture of Personalities, presents the voices of the First Ladies in novel format in an entertaining way. The Ladies talk with one another without regard to time and express their thoughts and feelings. Garfield's Train, although not specifically about First Ladies, relates the death of President Garfield in novel form. Thus it ties in with the topic of the Presidency.
To further aid writers, Feather has written an e-book, On the Road with the Old Gals. Here she gives hints on how to promote one's book through lectures/speaking engagements.
Are your topics ones you can specialize in, become an authority in, so that you are invited to speak and teach?