Gutenkarte was an amazing thing to see, mapping the locations within a book. However, it is only able to map and share public domain material.
Now GoogleBooks supports mapping of locations mentioned in books they've archived. See Lonely Planet USA for the locations mentioned in this travel guide. A great way to preview what locations the book focuses on and covers. (via hobu, via import cartography)
Or for fictional books, such as Neal Stephenson's Quicksilver it can help you decipher what happened where.
Google has the benefit of being able to store and share the text for currently copyright material, which is a definite benefit. But the integration of maps really adds to the information and understanding of the material.