on Jane Friedman site:

As writers, the happiness and relief are huge when we type THE END to our manuscripts. With each novel I’ve written, this has certainly been the case for me, but with my fourth manuscript, the sensation was slightly diminished because I knew by then what still lay ahead: writing the author’s note.

Now I love a good author’s note. I’m always eager to hear what writers say about their work. I find reading the note is a bit like chatting with the author. But for me, crafting the author’s note is an arduous task. For my historical YA novel, I spent close to thirty hours shaping and trimming down to a two-page piece.

So, what’s included in an author’s note?