The Gods Themselves, by Isaac AsimovRead about the other entries on the list.
I am recommending only the second part of this very variable book about the search for a safe, long-lasting source of energy by scientists in two different universes. Dua, who lives in the para-universe, is an unusual female of her species with unconventional desires and two conventional male spouses, Odeen, and Tritt. Reproduction for this threesome can go two ways. It may result in the birth of a Rational like Odeen, an Emotional like Dua, or a Parental like Tritt. But, eventually, the ecstasy of sex causes a permanent fusion of the three into one consciousness and a new being. Dua, Odeen and Tritt must figure out for themselves what they are and who they will become—and they must do it soon, while trying to communicate with scientists from our universe before they accidentally blow up our sun.
--Marshal Zeringue