Mirror Image

Mirror Image is the 19th novel in the Charmed (novels) series and is written by Jeff Mariotte. This novel is set between Season 4 Episode 8 Black as Cole and Episode 12 Lost and Bound as Cole is human, but he and Phoebe are not yet engaged. Because of this, it actually comes before Something Wiccan This Way Comes chronologically even though it was published several months after.

Summary
Phoebe Halliwell is stunned when her latest vision involves her younger half sister, Paige. The vision reveals to her that Paige is actually an imposter who will infiltrate the Power of Three and break it apart. However, Piper is dismissive when she learns of Phoebe's concerns -- so much so that Cole and Leo end up taking sides as well. Before long, all involved are on edge.

Paige senses that her sisters are acting strangely around her, so when she meets Timothy McBride, an attractive young stranger who purports to be a witch, she decides not to tell them. She wants to have something of her own, separate from her Charmed duties. Secrets are estranging the sisters from one another, and the timing isn't good. Women are turning up all over the city, dead by supernatural causes. The perpetrator may be connected to the sisters' past -- but if they can't work together, they might not be able to stop him.