At the end of the boardwalk is another elevator. Follow the pipe back from the elevator. Some of the pipe is missing! Oh, just extend it with the crank (good thing it didn't need water to power the crank to extend the pipe, whew!).
Make sure the water is channeled to the last elevator. Ride the elevator...the Linking Book back to Myst Island.
Go back to Channelwood Age -- Getting back
Go back to Myst Island -- major push