The Diary of an Immortal (1945-1959) by David J. Castello

During the liberation of the Dachau concentration camp in April 1945, a US Army medic discovers a Nazi immortality formula designed for Adolf Hitler. Inundated by death and destruction, he begins consuming the formula and traces its origin to a monastery in China where he learns some of mankinds oldest secrets and encounters an ancient race of immortals planning world domination.