Considered a Booth scholar, Brennan wrote the article "John Wilkes Booth's Enigmatic Brother Joseph", published in the Spring 1983 issue of Maryland Historical Magazine. Also among Brennan's works is The Three Versions of the Testimony in the 1865 Conspiracy Trial, published in 1983 and cited in the book Blood on the Moon: The Assassination of Abraham Lincoln.
As a Booth scholar, Brennan authored "The Mysterious Joseph Booth," which appeared in the Spring 1983 issue of Maryland Historical Magazine. Additionally, among Brennan's works is "The Three Testimonies in the 1865 Conspiracy Trial," published in 1983 and referenced in Blood on the Moon: The Assassination of Abraham Lincoln.