My next thought for Book Hack Day http://lanyrd.com/2011/book-hack-day/ is a favourite of mine when I was a child, The Voyages Of Marco Polo. I've no idea how I got hold of it in the first place, I assume that it was from my parents, but I loved this book. The edition in Waterstones has some nice maps showing modern day names for locations. Maybe a geo-based web site could bring it to life?!
The full text is available at Project Gutenberg as it is well out of copyright http://www.gutenberg.org/ebooks/10636