Sources for Lincoln laughing when he was shot 

I have several sources that write about Lincoln laughing when he was shot; 

"While waves of laughter echoed through the theater, James Ferguson kept his eyes focused on Abraham Lincoln...the president joined the crowd with a 'hearty laugh..."- The Darkest Dawn: Lincoln, Booth, and the Great American Tragedy by Thomas Goodrich

"...it is a comfort of sorts to know that the bullet hit Lincoln mid-guffaw. Considering how the war had weighed on him, at least his last conscious moment was a hoot."- Assassination Vacation by Sarah Vowell