The letter is shown in Flashpoint #1, it's the narrator captions. In order it would read like this.
There's only one thing I know about life.
I know some things happen by chance,
And some things happen because we make them happen.
Barry Allen was a man haunted by his past.
But when he became the Flash, he left the ghosts behind.
He found love. A Family. Friends.
Barry thought yesterday was behind him...
...But somebody wouldn't let him escape it.
When Barry came to me for help, I turned him away.
I'm not the hero of this story.
I'm a man who's been corrupted by his own unbearable pain,
I'm a man who has too much blood on his hands to be called good.
I'm a man who had nothing to live for...
...until the day I met the Flash.
The first time I met Barry Allen, I nearly killed him.
Like I said, I'm not the hero of this story.
After receiving and reading the letter Bruce Wayne breaks into tears over the letter and thanks Barry for being a good friend and for delivering this letter to him.