Stanley G. Weinbaum 
Worlds of If [PDF ebook] 

In "The Worlds of If, " author Stanley Weinbaum plays with a concept that has been at the center of countless science fiction tales before and after: how the most minute shifts can alter the past, present and future. In this case, nutty professor Haskel van Manderpootz invents a contraption that allows users to view alternate life paths that might have transpired if they had made different choices along the way.