National Treasure: Book of Secrets

On the run, Booth carried a small (6 by 3½ inches) red appointment book (for 1864) which he used as a diary. According to the FBI (who’d been requested to forensically examine the evidence), the diary is missing forty-three sheets,  totaling eighty-six pages. (See Lincoln Legends: Myths, Hoaxes, and Confabulations Associated with Our Greatest President, by Edward Steers, Jr., page 188)

Although the first entry is for April 14 - the day of the shooting - Booth likely penned his words between the 17th and the 22nd of April, 1865. The text appears to initially reference Booth’s original (but failed) idea - to kidnap Lincoln:

Until to day nothing was ever thought of sacrificing to our country's wrongs. For six months we had worked to capture. But our cause being almost lost, something decisive & great must be done. But its failure is owing to others, who did not strike for their country with a heart.

Then, Booth describes how he shot the president in front of a thousand people:

I struck boldly and not as the papers say. I walked with a firm step through a thousand of his friends, was stopped, but pushed on. A Col- [Colonel] was at his side. I shouted Sic semper ["Thus always to tyrants”] before I fired. In jumping broke my leg. I passed all his pickets, rode sixty miles that night with the bones of my leg tearing the flesh at every jump.Thus always to tyrants

Did he regret killing Lincoln?

I can never repent it, though we hated to kill: Our country owed all her troubles to him, and God simply made me the instrument of his punishment. The country is not what it was. This forced union is not what I have loved. I care not what becomes of me. I have no desire to out-live my country. [In other words, the Confederate States of America.] This night (before the deed), I wrote a long article and left it for one of the Editors of the National Inteligencer,  in which I fully set forth our reasons for our proceedings.

A week later, Booth was still on the run.