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.
Brennan, recognized as a Booth scholar, wrote an article titled "John Wilkes Booth's Mysterious Brother Joseph" that was published in the Spring 1983 issue of Maryland Historical Magazine. Furthermore, Brennan has produced another work called "The Three Testimonies from the 1865 Conspiracy Trial," which was released in 1983 and cited in Blood on the Moon: The Assassination of Abraham Lincoln.