Anti-Semite Adolf Hitler — born in Austria's Braunau am Inn in 1889 to Alois and his third, much-younger wife, Klara Pötzl — likely later sought to conceal that the family once lived in a Jewish-owned property in Urfahr near the Danube river city of Linz, the book reveals.
The letters also show that Hitler's mother, nearing death in 1907, was treated by a Jewish doctor who later escaped to America.
Hitler was already an anti-Semite in his youth, concludes Sandgruber, disputing claims that Hitler's hatred of Jews was forged after he moved to Vienna.
As a young man, Hitler moved to the city around 1908, aiming to become an artist, despite being turned down for study.
The latest findings are contrary to portrayals by Hitler's teenage friend, August Kubizek, who is often cited by other historians, maintains Sandgruber.
Adolf Hitler's only significant revolt against his father, notes Sandgruber, was to reject Alois' wish that he also pursue a civil service career.
"He wanted to be a free artist and not to follow in his father's footsteps," writes Sandgruber.
However, both father and son also shared "contempt" for authority and were anticlerical, although Hitler did not quit the Roman Catholic church, the historian added.
Alexandra Föderl-Schmid, who reviewed the book for the Süddeutsche Zeitung newspaper, noted that Hitler, "who attached so much importance to an ancestral passport and Aryan origin, had himself more than one gap in the family tree."
Even over 75 years after World War II, modern-day Germany still needs to rid itself of 29 legal or regulatory texts that allude to wording introduced when Hitler was in power, government-appointed anti-Semitism commissioner Felix Klein told news agency AFP last month.
Government critics, for example, are calling for the removal of the term "race" from Article 3 of Germany's constitution. Last year, Chancellor Angela Merkel declared herself open to such a deletion.
Being "Jewish" is DNA, not what church, temple, synagogue one attends!
Hitler's direct lineage is questionable.
I consider it almost impossible that Adolf Hitler had a Jewish grandfather or a Jewish origin. That is a propaganda story of the twenties or thirties. However, Alois was an illegitimate child, the father is definitely difficult to prove. It could also be an infidelity. The confusion arose because Johann Georg Hiedler did not have Alois legitimised as his son when he married his mother Maria Anna Schicklgruber. Either it was sloppiness or Johann Georg Hiedler really was not the father.
At the age of almost 40, Alois Schicklgruber initiated his own name change and registered Johann Georg Hiedler, who was already deceased at the time, as his father. This resulted in another name change.
The notary's certificate is remarkably sloppy, almost everything in it is wrong: the father's date of death, the first name and the spelling of the surname - Hiedler became Hitler. How this came about is not known. Adolf Hitler is said to have described the name change later as his father's best decision: Neither Schicklgruber nor the soft-sounding Hiedler would have worked as well as Hitler as party advertising.
Lineage sounds sketchy at best.