I see this pretty often on book titles and it bothers me that I can't figure it out.

Here's one I'm reading right now:

The Enthusiast: A Novel (P.S.)

 It's also printed on the top of the book's spine, but there's nothing in the book to clarify.

I'm willing to bet that this is something incredibly simple and I'll end up feeling like a dork, but I'm curious that way.

I'd never noticed this before, but a Google search brought up this result:  http://wiki.answers.com/Q/What_does_PS_mean_after_a_book's_title  .

Basically it is a Harper Collins feature, with some sort of special content (interviews, essays, photos) at the end of the book.

Yup, that's it. There's an interview with the author at the end of the book. I looked through the beginning of the book for a clue, but I never thought to flip to the back.

Thanks!

I see this feature on many US reprints of books.

I see this feature on many US reprints of books.

Ditto.  I've noticed it on several hardcovers that are printed in paperback for the first time.  Sometimes there will also be a little "P.S." triangle in the lower right corner of the front or back cover as well.  

Here is what the publisher said about these volumes when they created the line:

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.

I had to look that up too, PS, post script.  As in a letter.

yep, seems to fit "post-script" definition perfectly!  I love these newer books with book-club questions, author interviews, etc.