Wikipedia--and book reviews and essays--are often lifesavers if I'm in the middle of a book I'm really hating. Getting a firm handle on the plot helps a lot and sometimes a review will wake me up to some aspect of the book that was going over my head.
Oh, and one thing I thought I should add to my post above: Because I seldom quit books without finishing them, I'm usually careful about what I start. On GoodReads, I keep a To-Read list, which I limit to 10 books, and a Maybe-Someday list, which currently has 102 titles on it. For a book to get to spot #1 on my To-Read list, it usually means I've heard numerous good things about it from several sources over the course of about a year. So I usually find myself reading books I wind up rating 4 or 5 stars out of 5.
Of course, being human, I sometimes throw that careful procedure out and start reading a book just because. I did that with my current book, Boy, Snow, Bird by Helen Oyeyemi, and it's proving to be quite a challenge. I'm 60% done and expect I'll be rating it either 2 or 3 stars. I can never get more than 5% through it in a sitting, and then only if I'm fueled up on a couple mugs of coffee.
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