The book is finished and on the shelves in either electronic form or paper. Someone has bought the book. How do you as the writer keep the reader interested.
The first thing is knowing your targeted audience and using your words to reach the reader. This means if you're writing in a specific genre or even a cross genre, your prose is directed toward the mind of the reader.
A second way is to give the character or characters a time deadline to accomplish their goals. The characters who must accomplish a task in a short time will draw the reader to see if they will succeed.
A third way is to promise the reader action, emotion or adventure. Let them know the events you're writing about are important to the characters in the story.
A fourth is to find ways to keep the characters from reaching their goals. Obstacles that are real increase the tension and keep the reader turning the pages.
Chapter endings is the fifth. Ending the chapter with something that leaves a question in the reader's mind may cause them to turn the pages. We're all been lead down this path by skillful writers when we say "Just one more chapter" tonight and find ourselves continuing turning the pages.
A sixth is to have interesting characters with real problems to solve. A reader will invest in a character they can believe.