Because there will often be a good reason why a book reaches the top of the best-seller lists, and that is that it meets readers’ expectations. When that happens, it means happy readers. And happy readers are more likely to buy your next book or your older books.

Amazon will give you a good impression of what readers want.

Let us say that almost all the books in your category are 300 pages long. For some reason, and we will get back to this, you cannot hit that number of pages. Would it be better if your book was 100 pages long or 400?

The perfect length for your book is when you have said what you wanted to say. No more and no less.

If you write nonfiction, it is best to start with an outline following a structure that makes sure you do not skip important points. You can write this outline in a linear form inside the word processor of your choice. Or you can use a mind map tool and draw out the outline. The latter lets you add points easily if more comes to mind.

If you write fiction and do not outline beforehand, then your book needs to be exactly as long as your story takes you. Do not try to force a novel out of a story that cannot be longer than a short story.