I often like to read books without any idea of what I'm getting myself into, ideally without reading reviews, without reading book jacket copy, anything. I find that immersing myself in a story without any preconceived notions of what I think it will be about, or what I think it should have been about, is the best way to experience a book that's really good.
Plot is, of course, the big reason that I don't like to know anything about the book. I was spoiled for Ender's Game's plot, and although it's still one of my favorite books ever, I do wish I'd been able to read it without knowing anything about it beforehand. And then there's The Astonishing Life of Octavian Nothing, which is one book that is constructed in such a way that if I'd known anything about the book before reading it (including published reviews and Amazon reviews) I would have enjoyed it less than I did, as part of the fun is figuring out what is actually going on. (I'm really trying very hard not to mention C--- N--- V----- here! But that's another one where you really don't want to know anything, going in.) Sometimes even knowing that a book has a spoiler, or an unreliable narrator, is enough to spoil some of the fun of it for me.
There are some things I don't mind being spoiled for. Books that are just fluffy. Books I don't care about that much (apologies to anyone who loves them, but I read David Weber more for the fluff than for anything deep, so I'll frequently flip to the end to see what happened. Same with Cassandra Clare.) Romances (for the most part -- I appreciated not being spoiled for Georgette Heyer's Cotillion) -- usually it's pretty clear who ends up with whom from very early on in the romance.
Then there are some things I feel better off spoiled for. I reeeeally like happy endings, so if your book has a seriously unhappy ending? I need to know about it before I read it so I can psych myself up for it! (I don't mean if there's death, or rape, or trauma, or what-have-you, and the character comes to some terms with it -- I mean if it's actually a hopeless depressing ending, you know, rocks fall everyone dies.) Sometimes (this is more true for TV than it is for books, for whatever reason) I want to know if there's going to be a character death. Not always -- sometimes that can be one of the plot twists that I really appreciate -- but if it's telegraphed too blatantly and annoyingly, then I kind of want to know if I'm right. K.J. Parker's Engineer trilogy (which I don't necessarily recommend, unless you like depressing and misogynistic and engineers and Rube Goldberg -- if you do, well, it plays to all of those, and because I like engineers and Rube Goldberg I at least found it interesting) I had to flip to the end to see what happened to one of the characters, because if he was going to die I was going to be really annoyed. I also like to know if a book that starts strongly is going to end up wimpy. Or if it's going to make me angry (for whatever reason... being a curmudgeon, lots of things make me angry, from bad science to misogyny).
Books I wish I had been spoiled for (in case you DON'T want spoilers for these books, I shall not tell you why exactly, but comments are fair game if you want to know): Thirsty (Anderson) (I absolutely hated this book, but one of my friends loves it). Entwined (Dixon) (which mostly I quite liked). Girl in Translation (Kwok). I'd Tell You I Loved You But Then I'd Have to Kill You (Ally Carter). (Both of which had things I liked about them, and things I didn't.)
What books do you think are best read with or without spoilers?
For more book (music/TV) rants, you can also visit my personal journal.