The 3rd Swift Generations novel, this one featuring Phyllis Newton.
When Tom Swift Jr. suddenly disappears a week before his wedding, Phyllis Newton naturally goes off in search of him. The trail leads her first to a secret desert laboratory, and then to a period and a place in time where, in her search for answers, she encounters old friends, old enemies and a mysterious intelligence from beyond our universe. Having originally set out in search of an errant fiance, Phyllis soon finds herself holding the pieces of Existence in her hands.