Book Five in The Ammonite Galaxy series:

Six and Diva need time to get used to the changes in their relationship, but it looks as if everything will have to wait, because the two trimorphs have disappeared. Only the canths can dimly sense where they are, and what they see appears to greatly distress the animals.

Even so, two of the canths insist on accompanying Six and Diva on an insanely suicidal rescue mission, one which will take them right into the heart of the galaxy and the supermassive black hole which lurks there.