Yesterday's Tomorrow by Guy Rosmarin

Harvard historian Andy Spencer experiences a series of paranormal occurrences, leading him to believe he has stumbled upon a portal to World War II Czechoslovakia sixty years in the past. Whether it's real or a mere fabrication of his mind, he must find a way to cope with living in parallel realities without losing his sanity. Soon Andy learns that his two worlds are interconnected, and he finds a way to work it to his advantage. In the past, his unparalleled knowledge of history gives him a powerful insight into the future and helps him survive the war he's caught in. In the present, his experience on the other side of the portal sheds light on the riddle he has been struggling to solve his entire career, the true cause for the Third Reich downfall. But as he seizes the opportunity to piece together a puzzle that will redefine the greatest conflict in human history, Andy's two paths begin to converge, and the mystery becomes an existential threat on his life, the world he lives in, and everything he has grown to care for.