Sheffield's ''The Heritage Universe,'' which also includes Summertide and Divergence , reads as one continuous narrative, with each book ending at a good place to take a pause in the overall story. By this third volume, the group of humans and aliens we have followed throughout are now tracking the Zardalu. Thought extinct for thousands of years, a small number of these creatures--who are so deadly that they have become bogeymen in this future history--were preserved by the mysterious Builders, whom our heroes have been trying to learn more about from the beginning of their adventures, to little avail. One of the most interesting aspects of this series is the evolution of professor Darya Lang from the nervous academic on her first trip off her home planet in Summertide to a woman as or more capable of handling herself in a stressful situation as any of her companions in Transcendence . But, except for Lang and the android E.C. Tally, the characters are one-dimensional. Still, Sheffield's narrative is smooth, and the joys, pitfalls and dangers of exploration are conveyed well.