This is a hot topic among writers. Should a book be planned or is it best to just let the muse dictate all the twists and turns of your story's outrageous fortune?
I've seen folk come to virtual blows over it.
First off, I have to say I don't think there is a right way in any art form. Dogma kills spontaneity quicker than a dose of rat poison. And what works for one man will be anathema to someone else.
I used to be more of a confirmed pantser, ie flying by the seat of my pants. It's very much in the spirit of Nanowrimo which I've raved about in the past. The trouble is it's inefficient. You'll end up with a lot of dross and surplus material, scenes you don't need etc. But it is a thrilling way to write and I recommend it for anyone feeling blocked or jaded.
With each project I attempt I try to plan more. That's because each time you travel this road you see the pitfalls, the places where you wrote yourself into a blind alley. If only you had a more reliable road map to see you through the maze of ideas you could simply concentrate on the writing. So I plan. Or at least I try to. My creative mind is infuriatingly resistant to any limitations and tries to take the story off road at every opportunity. So I've learned not to plan too fastidiously and not to get upset if the story takes off in another direction I hadn't expected.