Son_Of_BDEC

This is NOT a bug. This is because you created the Linking Book in Age 3. The Linking Book only takes you to the Age that it was made in. Currently, your only hope of getting back to the overworld is finding a Star Fissure, it would seem. Unless there's been something added that I'm not aware of. (I think there may be a command to return you to Age 1, but I'm not sure.)