Night Moves by Randy Wayne White

Doc Ford has his share of secrets. One of them has returned with a vengeance…

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While trying to solve one of Florida's most profound mysteries, Doc Ford is the target of a murder attempt by someone who wants to make it look like an accident. Or is the target actually his friend Tomlinson? Whatever the answer, the liveaboards and fishing guides at Dinkin's Bay on Sanibel Island are becoming increasingly nervous—and wary—after a plane crash and other near-death incidents make it apparent that Ford and Tomlinson are dangerous companions.

What their small family of friends doesn't know is that their secret pasts make it impossible for them to seek help from the law. There is an assassin on the loose, and it is up to Doc and Tomlinson to find a killer before the grisly job is done.