If you want to read a book about German in the years preceding the rise of Nazis - one that also shows how a country's philosophy goes a long way towards determining its history - I highly recommend this book

https://www.amazon.com/Ominous-Parallels-End-Freedom-America/dp/0452011175/ref=sr_1_1?ie=UTF8&qid=1479192328&sr=8-1&keywords=ominous+parallels

The books mentioned by others are good books that explain the "what".....this is arguably the single best book that explains "why"