I read this in paperback around 2013; it was a pretty new printing at the time. it started with an alternate history '40s-era noir detective in wartime L.A. (or something similar). It flashed back to some nineteenth-century prospectors finding some sort of mystical pool that could clone/resurrect you; the two had a falling out and wound up in some centuries-long vendetta against each other as the world continued to change. There was some weird parallel dimension stuff with aliens like giant grasshoppers too; some grasshopper design was on the cover. I can't recall title or author for the life of me.
Pretty sure this is Liminal States (2012) by Zack Parsons.
Deep water rises. Abandon your spire. It is coming.
It is 1874 and Gideon Long is dying. Wandering the savage desert of the New Mexico Territory, he craves a last drink before he bleeds out. On the brink of madness, he discovers a place best left forgotten and makes an insidious bargain: escape his fate and incur a debt too great for one man. His country will pay the price over the twisting course of more than a century and Gideon will learn there are worse things to bargain with than the devil.
(Source: back cover)
Liminal States is the debut novel from SomethingAwful editor Zack Parsons, and it's extraordinary. It begins as a grim, relentless western novel that describes a doomed love triangle between a simple lawman, the twisted scion of an land-baron, and a woman who has married one but thinks she might belong with the other. After a botched train robbery and an epic battle, Gideon (the rich man's son) finds himself gutshot in the desert, led by a mysterious spirit animal to a mystical pool that dissolves him and then reincarnates him, young and whole and vital and immortal. Gideon goes back for the woman he loves, only to discover that she has died in childbirth, and, enraged, he kidnaps the lawman who was her husband and throws him into the pool, too. And now they are both immortal. Every time they die, they are reborn in the pool, over and over, locked in orbit around each other like twin suns being drawn into a destructive nova. This first third of the novel is dark and bloody and remorseless, a story of revenge and tragedy that doesn't let up, until...
From a reader review:
It's 1874. Gideon Long masterminds a botched train robbery. Gutshot and dying, he escapes into the New Mexico desert. Guided by visions, he crawls into a cave...and emerges younger, unscarred, his wounds miraculously healed. More curse than blessing, what he discovers there will forever change the face of humanity.
Now, hold onto your hat. The book that started as a western morphs into a crime/noir thriller set in 1950s Los Angeles. Long and his archenemy pursue each other for decades, prompting violent confrontations in the unending quest for vengeance.
It all winds up in the year 2006, where we are plunged into pure science fiction/horror as the spores hit the fan, and the world goes pretty nutso. And I haven't even mentioned the giant grasshoppers and flesh-eating mutants...oh, yeah!
Seriously, my words can't do it justice. I have never read ANYTHING like this before!