A book outline is a great way to encapsulate a lot of information into a convenient format. Books are full of big ideas, characters and stories, which means they are very difficult to distill into an outline. That doesn't mean the task is impossible if you follow the proper steps.
Note the major theme of the book. Most books are about something, and usually it's something pretty big. This is not the same as what actually happens in the story, rather, a major theme is what all the actions and people in the story represent. Finding this meaning is one of the hardest and most rewarding aspects of reading.
Create a list of all the characters, their relationships to one another and how they fit into the overall theme of the book. Detailing a list of characters helps give life and color to the outline. When you describe the story, this preparation will help fill in the gaps.
Write a detailed paragraph about each chapter. Adjust the amount of detail based on the number of pages needed for the outline. Some chapters will have to be distilled into a sentence or two due to space limitations. Include the major events in each chapter.
Discuss the symbolism of the story as it relates to the characters and events. This is another challenge for an outliner. Think about the story and the events and how they can be abstractly interpreted to mean more than what they say on the surface.
Make a conclusion based your findings. This will be a summation of the entire work and should be short and simple.