As anyone who's taken creative writing or literature classes already knows, books don't just end. There are several components that make up the "ending" of a book, each a crucial element of bringing a story to a satisfying conclusion.
The first of those is the Build-Up. This is the "beginning of the end"; it sets up the other components that come after it, and is an important part in making sure the ending makes sense and "feels" right.
- By now, do the readers believe in the characters?
- Do they identify with the heroine? (remember, I'm talking specifically about romances here; in other books, substitute the main protagonist)
- Is the two-step-forward, one-step-back process at a point where the hero/heroine are out of options?
- Does the pacing speed up?
> short, inter-cut scenes
> active prose (important throughout the book, especially important here)