The End of Time by Sharleen Nelson, Jeff Bolkan

In this electrifying finale to the Time Tourists Trilogy, time-traveling private investigator Imogen and her out-of-time boyfriend Simon face choppy seas as they navigate a global pandemic, a bitter and disgruntled former time traveler, shifting loyalties, and a universe straining under the pressures wrought by travelers attempts to alter the very fabric of history. With Simon sequestered within the bureaucratic ultra-secret time travel monitoring society, Imogen, along with her best friend Fletcher get stranded in the past when time travel suddenly and mysteriously shuts down. Simon, with his unique abilities, attempts to save the world and the thousands of people stranded in time. Relationships are tested, secrets revealed, and the mysteries involving both Simon's and Imogen's mothers come to light.