Elizabeth Bear has a nice post about what it means for a book to be broken.

So, a book can be"broken" in a lot of ways, but what it boils down to is that the narrative machine does not function. Not that it has dings on it, not that it grinds a little, but it just doesn't work. Examples are when the writer has to resort to TSTL (Too Stupid To Live) character actions or Deus Ex Machina to resolve plotlines. When characters must behave in an out of character fashion to make the book come together in the end. When the pacing is off, or the thematic resonances are set up badly or in a confusing fashion, so the sonar-image of a satisfying theme does not emerge from the echoes. When there is no click, at the ending, when it falls into place.

When you hold the book in your head, give it a spin on a fingertip, and you can see it wobble because the center of gravity is off somehow. (And I have no shit seen a wobble so big the book crashed and went bouncing across the room fixed by adding three paragraphs to the end. I am not kidding.)

This is a tricky tricky thing, by the way, because so much of it is subjective, and readers project a good deal of themselves into the narrative machine of a novel. They do, in other words, some of the heavy lifting. A reader who clicks with the inner squiddy nature of a book can patch a hell of a lot that's wrong simply by bringing his experience in to oil the gears and spackle over the gaps, to mesh with the machine.

But yeah, what I mean when I say broken is something deeper and more basic than a dent on the fender.

Crucial to note is that among other things, for a book to be broken means the book might be fixable. Of course, that’s only helpful if you’re the author and the book isn’t published yet, but still.