Sacred by Dennis Lehane
Amazon had referred to this novel as a modern day version of The Big Sleep - the only thing they seemed to have in common were the dying millionaire. Everything else, Lehane takes and completely turns around, ending up with a completely different story, which is much grittier and more interesting.
The dying billionaire in this novel hires Patrick and Angie to locate his missing daughter. He says that ever since her mother's death, his cancer diagnosis and a friend's death all within a short period of time, she has been struggling, and now she has disappeared. The investigator that was originally looking for her, who also happened to be the man that trained Patrick, has also gone missing. Patrick and Angie eventually trace Desiree to a cult, and from there, they end up flying to Florida to try and find out where she and their friend have disappeared to.
Things quickly get a lot more complicated as each discovery means that the whole story has to be looked at from a new angle. Lehane's novels all tend to have incredibly densely scripted plots with deeper and deeper twists and turns. In a later novel (I've been on a Lehane kick lately), Patrick says that he isn't a good chess player because he forgets to focus on the whole board, and this novel definitely shows that as well. Angie and Patrick are working with some information but as the novel progresses, they finally start to see the big picture, which paints a bleak and depressing picture of humanity.
On the plus side, Patrick and Angie finally get together in this one, both because and despite their shared past: they've known each other forever, Patrick's been in love with Angie most of that time, he married her sister, her ex-husband died in front of Patrick. A lot of complications, obviously. It was nice that they finally acted on their feelings, though, and had time for some actual happiness.