Mercy by David Baldacci

 This is the 4th in the Atlee Pine series. Atlee is an FBI agent who has spent years looking for her twin sister, Mercy, who was kidnapped when they were both only 6 years old. At the end of the last book, Atlee was finally getting closer to finding Mercy. When this book starts, Atlee and her trusty assistant Carol  (and I do love Carol) hit the road to follow up on some very promising leads.

The story keeps switching between Atlee's story, Mercy's story, and a rich man named Buckley who carries a grudge. It's a good progression to this compelling series.