Books about the craft of writing can be very helpful. If you use a good craft book you plan out your novel, you can save yourself a lot of frustration by developing characters and plot before you actually write. Jody Hedlund has compiled a great list of writers' favorite writing books.
But those books can also be a trap.
A writer I know recently showed me a new draft of a few chapters.
"What happened to Character A?" I asked. "I liked him when he was hardhearted and more interesting. He's all soft now. You already have enough sympathetic characters. You need a few harder ones for balance."
My friend told me that he had recently looked back over a writing book and made all kinds of notes about what his characters would or would not do, based on the principles in the writing book. Those notes changed his perceptions of his characters. And that was what led to my dissatisfaction with Character A's transformation into a sensitive New Age man.
"But you already knew your characters," I said. "You didn't need to think about them in a developmental way anymore. You can't let those writing books mess with your head. They're best at the beginning of the process, or as an emergency backup in the middle if your novel starts to fall apart. But yours wasn't falling apart. It was good!"
He agreed with me that he had been led astray by over-thinking, both in that change and in a subplot twist that grew way too big for its britches. He told me that he had had that funny feeling that all writers get when we know that something is not quite right, but we need an objective reader to help us identify the problem.
One of the things I love about this friend is that he also can tell ME when I've pushed something too far in a novel. That's what writing friends are for!
Have you ever been led astray by a writing book that you read at the wrong point in your writing process?