My wife recently read a book from start to finish in a single evening. The next night, she cracked open the same book and read it again. No, it wasn't one of my books, but that's OK. We have an understanding. She's allowed to enjoy books I didn't write.
In talking with her about it, I quickly realized what made the book so enchanting to her. It was the characters the author had created. My wife shared with me aspects of their lives, dialogue, relationships, backstory. She talked about them as if they were people she'd known her whole life. What we didn't talk about was the plot of the book. It almost seemed irrelevant to her.
Well-developed characters can not only make a book readable, they can make it re-readable. Think about it. The allure of a mystery that relies on clever plot twists and the unknown to hook readers doesn't quite have that same allure once the twists are revealed and the unknown is known. You may have enjoyed the book immensely, but chances are you aren't going to read it again.
The exception to this would be the same book, but with extraordinary character development. Then the book has an appeal that extends beyond the mystery it reveals. You may re-read the book just to reconnect with the characters you miss. You know the mystery within, but that no longer matters because you're a fan of the characters.
If you want to write a book that is re-readable, the part of your craft you need to develop is character development. It is the one aspect of storytelling that keeps readers coming back over and over again.
Richard Ridley is an award-winning author and paid CreateSpace contributor.