I was thinking about the strategy of giving an excerpt to the reader after they have finished the book (for various diabolical reasons and all that), but then I began to wonder what kind of excerpt would be most appealing to the reader. Would you like a scene that occurred within the story line, but was not included in the story proper? Would you like one that occurred after the story line? Or would you like one that occurred before the story line?

Personally, I would like one best that occurred before the story line. It wouldn't mess with the sense of the finality of the book like one that occurred after the story line would, but it would satisfy some of the reader curiosity perhaps, about how the characters got that way and so on.

As for one from the middle of the story line...I don't know. It seems to me that there would have been a reason why the scene wasn't just put into the story proper. It might be interesting, but the reader would have to really like the story to be as interested in it. At least that's my impression.

But what do y'all think?

I think I would like one before or after the story. An excerpt from the middle of the story might be a bit hard to connect to or get into. I don't know...just my opinion.

I think it depends entirely on the story. Is it standalone or part of a series? Are you leaving it open for a sequel? If you are planning on more books, you want to consider how your bonus scene will draw your reader into the rest of the series.

Prequel scenes are versatile, especially if you don't plan on writing a full prequel. As one idea, it's a lot of fun to take a bit of backstory that was mentioned in the novel and write out what happened in detail.

After-book scenes are a little more difficult in terms of leading into a sequel if there is one, or affecting the book's climax, as you mentioned. However, sometimes it's sweet and satisfying to get a taste of what happened to the characters we love so well. Pixar is pulling this off with the Toy Story short films they've made showing some of the trouble the toys are getting into at Bonnie's house. As a devoted fan of the franchise, I find this very satisfying. It gives us a taste and allows us to play with our beloved characters again, but it's not too involved so it doesn't wreck the beautiful finish of Toy Story 3. Doing a full-out Toy Story 4 movie, however, would ruin everything and stretch the concept too far. (Please don't, Pixar. Just don't.)

I personally am always a fan of "deleted scenes." They work quite well for movies, and I think they have potential for books as well. I released a few such things from Red Rain on my blog as promotional material. They were true "deleted scenes" in that they were cut before editing, so they weren't perfectly accurate anymore. But it was cool to see a taste of what the pre-edited version of the story was like.