Alright, so you're looking for the Atlee Pine series reading order that actually follows her main storyline. It's a bit tricky because the first book sets up her personal mission, but the real serialized arc doesn't fully kick in until later. For a complete picture, you absolutely need to read them in release order: 'Long Road to Mercy', 'A Minute to Midnight', and 'Daylight'. The fourth book, 'The 6:20 Man', is a David Baldacci book but it's NOT an Atlee Pine novel—it's a different series altogether, and I've seen a bunch of people get confused by that. The core trilogy is what gives you Atlee's journey from an FBI agent haunted by her twin's abduction to someone closing in on the truth.
That third book, 'Daylight', is where most of the lingering threads about her sister Mercy get resolved. It's the payoff for the whole 'what happened to my twin?' mystery that drives her. If you skip to it, you'll miss all the groundwork and character development that makes the ending land. I tried reading 'Daylight' first out of curiosity and it felt completely disjointed, like starting a movie at the third act. The emotional weight just wasn't there without the build-up.
Some argue you could read the short story 'No Time to Lose' (from the anthology 'FaceOff') anytime, since it's a crossover, but it fits best after the first book for tone. Honestly, sticking to the three main novels in order is the only way the arc feels complete.