So, place your bets: how many seconds after the final Harry Potter book goes on sale before someone opens their copy, turns to the final pages, scan them and then yells out, "Hey, Harry dies!" or "Voldemort gets eaten by a squirrel!" or "George Weasley is Harry's real father!" or whatever the ending
The rest of the over-eager kids and kidults in the queue would probably find that pretty annoying. They might even hunt down the plot-spoiler and rip him or her apart limb from limb.
Even for Potterless novels, some people buy a book but find it hard to resist flicking ahead to read the last few pages to find out the ending. But that's like wanting to know how a magic trick's done, it's like junk food, it's like unwrapping your Christmas presents early - it's tempting, it gives you a quick buzz, but then leaves you with an empty, hollow feeling afterwards.
If, as an author, you'd prefer your readers don't do this, if you'd like them to build up their appetite before gorging themselves on the final pages of your carefully-crafted novel, then you should publish your book as a 'Locked Last Chapter' book.
In Locked Last Chapter books, the pages of the last chapter or so are tightly shut between two metal plates (or maybe just laminated card if cost's an issue). The outer plates, which extend slightly out from the rest of the pages, are held together by a combination lock.
On the preceding page, after the end of that chapter, there's a one-page quiz where the reader can find out the code to unlock the final chapter. The quiz ensures the reader has read enough of the preceding plot to be allowed to open the lock.
This could also be a good selling point for the first few authors to publish in this format - "The ending of this book is so exciting, we're keeping it secret! You have to read the book to find out the ending!"
An example question for the quiz might be:
"Choose the correct answer to find the first digit of the combination:
While he was tripping out on the 'magic potion' his brothers brewed for him, Ron ate:
A bag of every-flavour sweets = 0
His dinner = 1
His essay = 2
His wand = 3
His sock = 4
Hermione's cat = 5
Harry's owl = 6
Hagrid's dragon = 7
Snape's hat = 8
Dumbledore's beard = 9"