Conquistador — S.M. Stirling

Oakland, 1946. Ex-soldier John Rolfe, newly back from the Pacific, has made a fabulous discovery: a portal to an alternate America where Europeans have never set foot--and the only other humans in sight are a band of very curious Indians. Able to return at will to the modern world, Rolfe summons the only people with whom he is willing to share his discovery: his war buddies. And he tells them to bring their families....

Los Angeles, twenty-first century. Fish and Game warden Tom Christiansen is involved in the bust of a smuggling operation. What he turns up is something he never anticipated: a photo of authentic Aztec priests decked out in Grateful Dead T-shirts, and a live condor from a gene pool that doesn't correspond to any known in captivity or the wild. These finds soon lead him to a woman named Adrienne Rolfe--and a secret that's been hidden for sixty years...