This is such a simple question with such a complex answer. And that's precisely what makes artists' books so fascinating. Here's Simon's short answer:
For more information, check out how these organizations have tackled the question:
You can also try these great videos:
Make sure to take a look at visual artist and author, Angela Lorenz's answer to the question.
Here are a few resources to help you get started in making your first artist book!
Little Book of Book Making: Timeless Techniques and Fresh Ideas for Beautiful Handmade Books by Charlotte Rivers
Making Books by Hand: A Step-By-Step Guide by Mary McCarthy and Philip Manna
Creating Books & Boxes: Fun and Unique Approaches to Handmade Structures by Benjamin Rinehart
Smithsonian Libraries on You Tube