Here is what he wrote according to Paine in the Twain biography:
Clemens, coming to say good night, saw a little group about her bed,
Clara and Jean standing as if dazed. He went and bent over and looked
into her face, surprised that she did not greet him. He did not suspect
what had happened until he heard one of the daughters ask:
"Katie, is it true? Oh, Katie, is it true?"
He realized then that she was gone.
In his note-book that night he wrote:
At a quarter past 9 this evening she that was the life of my life passed
to the relief & the peace of death after 22 months of unjust & unearned
suffering. I first saw her near 37 years ago, & now I have looked upon
her face for the last time. Oh, so unexpected!-...! was full of remorse
for things done & said in these 34 years of married life that hurt
The New York Law School, Mendik Library