Reddit mentions of 50 Common Core Reading Response Activities: Easy Mini-Lessons and Engaging Activities to Help Students Explore and Analyze Literature and Informational Texts

Randomly, I just finished that book a few weeks ago, and I have a few insights. First and foremost, knowing who the killer is doesn't actually tell you much about the book. It doesn't tell you how everyone dies, or even if everyone dies, so there's still lots to be gained by finishing the book. If the kids are caught up on having the main mystery spoiled for them, point out the other mysteries that they still don't know the answer to. Keep going back to the "Little Indians" poem. Have them make predictions about how each person is going to die, or if they're actually going to live. Do some kind of betting pool, or give out prizes for correct predictions so that they've got some skin in the game.

You can also talk to them about the difference between reading a book "for funsies" and reading a book for class. When you read for fun, you kind of plow through and don't think too much about the author's craft. When you read for class, your main question shouldn't be, "How does the story end," they should be, "What tools did the author use to make me feel this way about this passage/character/story," or "What quotes help me to understand this character," "What is the author's purpose?" or similar.

Next, I would advise that you never go into a novel unit without an end game in mind. The end game is not to get to the end of the book, it's whatever major ELA concept are you trying to get the kids to understand. That way, if someone spoils the ending, you can tell the kids yeah we know how it ends now, but we're studying this book for story structure, or characterization, or theme, or whatever standards you're trying to hit. It doesn't matter if you know the ending. After you prep them, track this standard as it appears throughout the book. For example, when I taught Freak the Mighty to my 7th graders, I used it to teach characterization. We evaluated how the author presented the characters throughout the book, what the characters said about themselves, collected quotes as evidence as we read, and at the end we wrote an essay about whether or not the characters evaluated themselves correctly.

For And Then There Were None, you could try and get them to see how the author builds tension over the course of the story, but I tried that with my eighth graders one year using "The Monkey's Paw" and even with a short text they couldn't quite grasp it. You could probably have a lot of fun doing a characterization unit though. Do some character projects like the Split Open Mind where students analyze the difference between how the character feels on the inside vs the side of them they are displaying to the other guests, or have them do character poems for different characters (this is a pretty good template which you could adapt for your needs - do a google search, there are tons of examples), or a character sandwich (different colored paper for each sandwich ingrediant, on one side of the ingredient is an adjective describing the character, on the back side is a quote from the text, staple together), or character colors (write the character name on a peice of paper in a color, and explain on the back why you chose that color for that character using evidence), or any other character analysis project you like. Having an artistic aspect might get your kids engaged in the book again, and they'll find themselves performing rudimentary analysis without even knowing it!

For additional ideas, I just picked up this book, and so far it's got some interesting stuff in it that you might be able to use. Also look for books on teaching book clubs, they've usually got recommendations for great projects that are engaging and help kids get deeper into the text.

Hope I've thrown something at you that might help! I hate it when you have that one kid that wants to be a jerk and spoil it for everyone else. When it happens to you again (because even if you ask them not to, it will probably happen again), don't confirm the spoilers. Just shrug and say, "I don't know, it could be!" with a significant look. You have enough authority that the kids may not believe the first kid if you don't confirm what he says. Then have a stern conversation with that knucklehead after class. That was a power move he pulled on you, trying to disrupt your unit. It wasn't cool, and it wasn't alright.