My answer to the original question: This is something that an author as author shouldn't be concerned with; if you're self-publishing, this is something to consider when you're wearing your "publisher" or "marketer" hat. In my experience, it can work really well, it's usually far better than a bunch of blank pages (and filling up unused pages in a run was how the practice got started, I'm fairly sure), or it can be incredibly tacky. Also note that it doesn't have to be a "sneak peek" of 
your next book; in fact, I think I've seen as many short excerpts of other recent books published by the same publisher (sometimes by the same author) than of sequels. I think you don't see them in hardcovers as much for marketing reasons (about which I could only speculate) as any other. And since ebooks automatically give a "sneak peek"/"try-before-you-buy" option, I think it'd be slightly tacky and an unnecessary use of effort to duplicate it in a previous ebook---but a list of "other (or later) books by this author" or "author's recommendations" might be a nice touch.
Oh, and I don't like the term "sneak peek" for these things---"preview" (or "exclusive preview" if the excerpt won't appear anywhere else except in the book itself) is better IMO, and there are other phrasings that I've seen used but can't remember.
Lycanis Mimetes wrote:
As a reader I love/hate that...I like it when you can get past the cliffhanger of the ending of the last book, and it helps tide me over till the next one comes from the library...but I hate it because sometimes it spoils the cliffhanger and takes away some of the "magic" of getting the next book, or it sets an even worse cliffhanger (which is good marketing and I don't hate it as much as the other problem, where it spoils the cliffhanging part). So I think it might be good to have a short excerpt, but you need to be careful and know what you're aiming to do with it.
Personally, I'm opposed to cliffhanger endings as such; I much prefer the some-things-resolved-others-opening-up ending that leaves the reader 
satisfied but wanting more---cliffhangers only work as an incentive once, and can backfire (sometimes readers' aversion to overt manipulation of their emotions is stronger than their desire to find out what happens next).
Aleena Mimetes wrote:
Well, I see it done with lot's of "big name" fantasy writers like Batson, Paul, (who wrote On the Edges of The Dark Sea of Darkness?) and Decker.  
However, classic authors like Lewis and Tolkien didn't add excerpts. 
Personally I don't read them, but I don't think it seems professional.
Like I said, this is a 
publishing decision; I've seen it in some (paperback) editions of Tolkien (and I think Lewis too).
The "unprofessional" "smell" to it may be part of why you usually only see it in paperbacks, since those have carried that stigma for decades 

.
Constable Jaynin Mimetes wrote:
Lewis and Tolkien wrote before such things existed. That's why they're classic.
I think I've see them in some of my parents' paperbacks from the '60s, which were certainly contemporary with the Inklings. And the Ace paperback editions of 
The Lord of the Rings (which were arguably "pirate" editions, but were, I think, technically legal under U.S. law at the time) might have even had these "previews" themselves.
(One thing from that era that I'd like to see done in the ebook era---other than the prices, which are about the same as paperbacks were then---is "doubles," books bound back-to-back and sold for the price of one.)
Constable Jaynin Mimetes wrote:
It just seems a bit tacky to have a nice collection of all the books in a series and have two versions of the first chapter of most of them... or is that just me?
In my experience, it wouldn't be "two versions," just "two copies"---the point is that you're getting a peek into the next book; it's not like a movie preview, which is heavily edited for maximum effect and thus is less reliable as a guide for whether you want to buy it. But this is, I suspect, part of why we usually only see "sneak peeks" in hardbacks---paperbacks have a stigma of being "disposable" (and I've seen people come to friends-of-the-library book sales and buy an entire rack of paperback romance novels, not caring that they themselves donated many of them months or years ago), while hardbacks are (ideally) built to last, and priced accordingly.
Lycanis Mimetes wrote:
No, I agree...I think it might be best to go with an epilogue that sort of resolves the first part of the story and at least hints at the next one...I don't know. 

In my opinion, it's best to end a story when you've resolved the threads it was focused on and begun to transition to the next one, full stop. An "epilogue" should only be needed if there's not 
going to be a "next one" to resolve any remaining threads.
Arien Mimetes wrote:
If you're going to have the first chapter of the next book at the back, I think it should probably be as close to exactly the same as possible to the actual first chapter of the next book. So if that's going to be a problem, I'd say don't do it.
I agree---except that it doesn't have to be the first 
chapter, so much as the first however-many pages---preferably breaking at a reasonable stopping place, but some authors' chapters are either too short or too long for the available pages. (As I alluded to above, a book---including all necessary front- and back-matter, is or at least used to be published on one or a few really big pieces of paper, which are then folded, attached together, and cut to form the final book. Depending on the relative size of the original paper and the final pages, this gives you some multiple of either 2, 4, 8, 16, 32, 64, etc., pages---and any that aren't used show up in the final product as blank so the publishers typically find some "advertising" use for them in paperbacks, and I think adjust the margins and such to avoid them as much as possible in the "more professional" hardbacks.)
Lycanis Mimetes wrote:
you need to keep that there for...well, as long as possible, unless you put it as a limited time thing...
In my opinion online content should in general be permanent, so links to it, bookmarks, and such work indefinitely---even if you change blogging platforms, or something. (Finding that five-year-old or ten-year-old bookmarks don't work when I suddenly need the information they linked to---or want to read the story again---is 
quite annoying.)
Constable Jaynin Mimetes wrote:
What do you think about the idea of including the sneak peek in ebooks but not in the print version?
Like I said, that's ... backwards. If the reader is already online, it's not very jarring to see a link to the "read inside" page of the sequel, while it's more trouble than it's worth (and arguably padding the word count, like an unnecessarily-verbose summary of the previous books at the beginning) to duplicate that content in the previous ebook, but in paper many readers will just skip a "start reading the next book online!" link but will "take the bait" of a "teaser" that's right there in the book.
Lycanis Mimetes wrote:
Or you could put a special connecting bit of story at the end of each book that isn't in the next one at all...sort of like a pre-prologue or post-epilogue. 

 (I'm actually serious about that idea, but I'm not sure if it's totally ridiculous or not 

)
 is something that feels like a gimmick to me, and so would make me less likely to try your book. If it's relevant, it should be part of the story 
per se, not created to include in some editions as a marketing gimmick. (But putting in a note "exclusive Web content on the author's website" and putting things like this on your blog feels less gimmicky, probably because textbook publishers have been doing it for years.)