Shadow out of Time by I.N.J. Culbard
Nathaniel Wingate Peaslee is a professor of political economy at Miskatonic University. Following a fainting spell in 1908 he returns to his senses in 1913 with no recollection of the last five years of his life.
As he endeavors to discover the truth about his lost years, he becomes increasingly tormented by vivid and disturbing dreams; dreams that will lead him on a journey through time and space to unlock the secrets of the universe.