From the top 100 free list
The New Madrid Run
The New Madrid Run is a tale of desperate survival on an altered planet: In the aftermath of a global cataclysm caused by a shift in the earth’s poles, a handful of survivors face the terrible elements of a changed world as they navigate a battered sailboat from the ruins of Florida into the hills of Arkansas via a huge rift in the continent (the New Madrid fault). They survive fierce storms and high seas pirates only to make landfall and discover the greatest challenge of all.
Quicksilver (The Forensic Geology Series, Prequel)
A young man disappears in the wilderness of the California mother lode. He leaves behind a gold-flecked rock and a vial of liquid mercury. He is a misfit in the modern world, a throwback to the Gold Rush days.
A venture capitalist--whose gold country is Silicon Valley--hires forensic geologists Cassie Oldfield and Walter Shaws to track his missing brother.
Following one of the 'lost rivers' of California, Cassie and Walter plunge into the dark history of the legendary lands, into the dark past of the brothers, into a poisonous sibling feud that threatens both lives and the land.
And the question then becomes: which brother is on the hunt?
[#60 - I think I got another book in this series for free some time ago. I don't think I read it.]
Blood and Justice (A Private Investigator Series of Crime Thrillers)
When sixteen-year-old Jenny James goes missing, and the local police are unable to find her, the girl's frantic mother hires private investigators Jake and Annie Lincoln to search for her daughter.
When the body of Jenny's boyfriend is discovered, the mystery of her disappearance deepens. Shaken out of their comfort zone of Internet searches and poring over public records, the couple soon find themselves facing the frightening possibility they are looking for the latest victim of a serial killer.
As more bodies pile up, the town is gripped with fear. It seems no one is safe, and the Lincolns race to solve an impossible puzzle before they become the killer's next victims.