The Frozen People by Elly Griffiths

Cold cases are a lot easier to solve when you can travel back in time to find new evidence—unless, that is, you get stuck in the nineteenth century.

Ali Dawson and her cold case team investigate crimes so old they’re frozen—or so their inside joke goes. Ali’s work seems like a safe desk job, but what her friends—and even her beloved son—don’t know is that her team has a secret: They can travel back in time to look for evidence.

So far Ali has made trips only to the recent past, so she’s surprised when she’s asked to investigate a murder that took place in 1850. The killing has been pinned on an aristocratic patron of the arts and antiquities, a member of a sinister group called “The Collectors.” She arrives in the Victorian era during a mini ice age to find another dead woman at her feet and far too many unanswered questions. But when her son is arrested, Ali attempts to return home only to find herself trapped in 1850.