The Threshold — Marlys Millhiser

A doorway to the past reveals a warning for the future in this time-travel adventure by the author of Michael's Wife 

Just released from prison after serving time for a false drug charge, Aletha Kingman decamps to Telluride, Colorado, for a fresh start. One day when she's sketching an abandoned miner's shack, she encounters a young girl, Callie, who's been transported from the turn of the century. Aletha follows Callie back in time to the rough-and-tumble mining town where the impoverished girl faces a future of disrepute and her brother Bram is caught in the bloody conflict between hard-working miners and their bosses. Together with her newfound friend, the enigmatic Cree, Aletha is determined to use her foresight from the future to make a difference in the past.
 
Suffused with the social and political history of the American West, The Threshold crosscuts between the disparate worlds of glitzy modern-day Telluride and its past incarnation as a gritty, turbulent mining town, brilliantly posing the question of whether we can -- or should -- alter the course of history.