Sweeter Than Wine by L. Neil Smith

In 1944 an American Lieutenant gets trapped in a basement when the Germans counterattack. He's not alone. He and the beautiful Romanian pilot fool around while they wait for the Allies to take the village back. He catches an STD. Suddenly he is allergic to garlic, silver, and sunlight. Eighty some years later he still looks twenty-four years old and is living in Colorado working as a PI. He hasn't seen his lady love since they got separated in 1944. Then she shows up and the adventure begins.

L. Neil Smith has written a fun novel here. He avoids his recent problem of being too preachy, but still manages to push his libertarian ideals. There's lots of guns as expected. It a good book and I highly recommend it. It doesn’t appear to be available in paper but you can find the ebook here.