What is a "P.S. Edition" of a book?

It's just a trade pb edition with some extra material in the back.  Often an author interview, boook group discussion topics, etc. 

Here is the info one our fabulous TGs Sianeka found:

Harper Collins Press Release

HARPERCOLLINS ANNOUNCES THE LAUNCH OF P.S.?,

New York, NY (July 29, 2004) HarperCollins announced today that its Perennial imprint is launching a new trade paperback program called P.S.. Titles in this program will contain an additional section at the back of the book with behind-the-pages insights, interviews and more. The extra content in this section will take these titles beyond traditional reprinting and offers readers added value with their paperback purchase.

Similar to DVDs which regularly include outtakes and behind-the-scenes footage from feature films, each P.S. section will be unique; the extra 16-pages will offer readers an in-depth look at both the author and their book. The P.S. section will use interviews, essays, articles, photos and illustrations to explore the authors' sources of inspiration. P.S. may also contain topics related to the book's subject matter and show the impact a book has had since its publication.

"P.S. was conceived by our HarperCollins UK colleagues and we immediately fell in love with the idea of giving our readers more. We are always looking for ways to be innovative and help our authors reach a wider readership. P.S. will do just that," said Michael Morrison, Executive Vice President of HarperCollins.