Mysterious Big Dumb Objects have been found on Titan. It appears that they have been placed there by the “Others”, who will return to threaten mankind. Although somewhat disjointed and lacking focus, I nevertheless enjoyed this story of mankind under an unknown threat. The novel is vaguely connected with the Orion series.
Widely considered to be Bova’s masterpiece, I never really figured out these books (and yet for some reason I read three of them). Our titular hero loses his memory, fights evil as he jumps back and forth in different eras of past and future. These jumps are more or less out of his control. He constantly loses and regains his beloved. The first book is decent, but after that it’s really just rinse and repeat and they blend into each other in my memory. I gave up after book three.