After he sails away, go back down the stairs, turn the switching
lever to the
left to open the force field. Go out through the left exit, down
the stairs to
the Tomahna linking book. Click on it. It will transfer into
your inventory.
Click on it to bring it up, open it and return to a relieved and
happy Catherine
and Atrus, as they receive from you the precious Releeshan linking
book.

So it is said: "All's well that end's well."