Crusader's Cross - James Lee Burke

In the summer of 1958, Dave Robicheaux and his half-brother Jimmie are just out of high school. Jimmie and Dave get work with an oil company, laying out rubber cables in the bays and mosquito-infested swamps all along the Louisiana-Texas coastline.
They spend their off time at Galveston Island, fishing at night on the jetties, the future kept safely at bay, the past drifting off somewhere behind them.

But on the Fourth of July, change approaches in the form of Ida Durbin, a sweet-faced young woman with a lovely voice and a mandolin. Jimmie falls instantly in love with her. But Ida's not free to love - she's a prostitute, in hock to a brutal man called Kale, who won't let her go.
Jimmie agrees to meet Ida at the bus depot, ready for the road to Mexico. But Ida never shows.
Dave and Jimmie want to believe she skipped town, but they know, deep down, that Ida Durbin never got to leave.

That was many years ago - before Dave Robicheaux began his long odyssey through bars and drunk tanks and skin joints of every stripe. Before the Philippines and Vietnam.
Now, an older, well-worn Dave walks into Baptist Hospital to visit a man called Troy Bordelon, who wants to free himself of a dark secret before he dies. A bully and a sadist, he has a lot to confess to - but he chooses to talk about a young girl, a prostitute who he glimpsed briefly as a kid, bloodied and beaten, tied to a chair in his uncle's house. Dave realises he can't let the past go. Ida's killers are still out there. So he begins his journey into the past - back to the summer of 1958 and a girl called Ida Durbin.