What Price Paradise? — Phillip Ellis Jackson

In this final book in the Timeshift Trilogy, a brilliant maverick scientist discovers a way to insert a person from the future into the still-living past. Keith Maravich, life-long friend and brother to Carson Gilmore, travels back to May 1999 to alter the course of human history, which placed mankind on its path to extinction. He is to save Alicia York, a scientist at the University of Chicago, whose murder ends her promising research into the creation of new forms of bio-chemical life; research, which, if completed, would have eliminated the threat from the virulent, omnipresent ash plaguing the 25th century. As the critical day approaches, Gil and Keith meet at the designated time and battle one another five hundred years before they were born, while the fate of humanity hangs in the balance.