The theme for your book is the central point that the entire plot revolves around. If you don't have a strong theme that can carry through to the end, holding a reader's interest will be impossible. Often, the difference between a book that is sold and one that ends up on the slush pile is the strength of theme. Before you even start your book, it is vital to develop a central theme.
First, you need to plot out the entire book, start to finish. This outline will be useful in helping you finish the book, and can be used if you get stuck in the middle of a chapter. This outline will help you develop the central theme in your book. What is the main premise of your book and how will you work towards resolution?
Consider also the genre you are working in. For example, if your book is a murder mystery, you will need to create a theme that supports the plot elements. If your book is an action adventure, the main theme should be the quest. Whenever you are stuck, you can always fall back on your theme to pull you through.
It has been said for centuries that there are twelve main types of stories – or themes if you will. These themes follow a set formula. Using the above example, an action adventure story typically revolves around a quest that the characters must undertake to reach their desired goal. A murder mystery revolves around clues given to the reader and the main character's attempts to solve the mystery.Your theme must be strong, it must be believable, and it must follow the set formula for your genre. While there is nothing wrong with departing a little or mixing thematic elements, at its core, you must have a theme that can carry the story from beginning to end.