Illustrations In books: enhancing or distracting?

When are illustrations in a book a hindrance and when does it enhance written content? That is the question I started to think about after watching an interview Debbie Millman did with Malcolm Gladwell and the design team that transformed Gladwell's trilogy, The Tipping PointBlink and The Outliers with illustrations thus producing an illustrated set of his books. Well thought out Illustrations that complement and not distract the readers are a good thing and can even add value to the book. In today's world of technology books are more and more falling into the grip of the screen.  As technology progresses, the physical book has to become more than just mere words on a page.  Those pages must come alive.  

An example of a book that combines illustration with text successfully that I've come across is Elizabeth Gilbert's, The Signature of All Things. The book features some colorful illustrations of plants and some black and white illustrations of plant studies In the beginning of each part.  For those who have read the book, this makes sense, being as the entire novel centers around the main character's passion for botany. The design of the book, simple as it may be, adds to that feel of old illustrations of plant studies and of books of that time period. With this particular novel, the illustrations served it well.  However in other cases sometimes a story (fiction) left to the imagination can be a great thing.  That is what is so fantastic about books, it awakens our imaginations.

Another example of a book that uses illustrations to enhance the psychical book is I Wonder by Marian Bantjes.  As a matter of fact the illustrations are the main focus for this book.  Bantjes is a graphic designer and fully utilizes her love of illustration, photography and design in this book and she went all out with using gold and silver foils on black satin cloth for the cover. In this case, having the physical book is a much better experience than viewing the book on a screen.

Besides the fact that illustrations can add value to the physical book, I must also make the point that from the caveman era till now we have been visual beings and so if Illustrations can complement and enhance the written content, I say go for it! It'll make for a much better experience.