Murray Leinster 
Runaway Skyscraper [EPUB ebook] 

Arthur Chamberlain, is an engineer who works at the Metropolitan Tower in Midtown Manhatten. When the sun suddenly begins moving backwards in the sky he is the only person who realizes what this means. A flaw in the rock beneath the building has caused it to subside, but instead of moving in space, the building is falling backwards into the past. When it finally stops he and the other 2, 000 people in the building find themselves stranded in a dangerous pre-Columbian Manhattan. Can he figure out how to get them back to their own time? More importantly can any of them survive long enough to get back home?