In the year 2010, this separation between print and web content seems very antiquated. Give subscribers access to all the same content that's in the book, even if it costs a bit more; then people can decide whether they want to buy the book or the online version. Better yet, include free access to the online version for people who buy the book (may need to adjust purchase price, of course). Magazines do this all the time - the print subscribers can access the articles and/or supplemental material online. I'm not in the publishing world, but this doesn't seem like it would be hard.