Bill Belichick's most important biographical detail is, of course, his relationship with his father. I have discussed that, with quotes from The Education of a Coach, in this game post.
Here's the rest, as told by Halberstam:
The name was originally Bilicic, and it was phoneticized or Americanized, not, as happened with so many immigrant families, at Ellis Island, by immigration officials reluctant to master the names from the old world, with their weird mixtures of vowels and consonants, but rather by a first-grade teacher in Monessen, Pennsylvania. That teacher, told by Anna Bilicic, Steve's oldest sister and the first family member born in the United States, that her name was Bell-uh-chik, wrote it down as Belichick, thereby earning the undying hatred of Mary Barkovic Bilicic for corrupting the family name.
The values of that era and of that particular ethnic culture were basic. You worked hard. You saved. You did not waste anything. If possible, you grew your own food. You did not complain. You did not expect anyone to do anything for you. Discipline was not so much taught as it was lived, as an essential part of life for which there was no alternative. It was, says Bill Belichick, 'a Draconian unsparing world.'
The entire region of western Pennsylvania and eastern and central Ohio was great football country, both high school and college football. Everyone seemed to care passionately about the game. This was, after all, a part of the country where tough men endured great physical hardship to earn a living--only the strong succeeded, and not surprisingly, they produced big, strong, athletically gifted children, who had no fear of ferocious physical contact--indeed, they seemed to relish it. In the era before the coming of the great black athletes, when power was blended with speed and the game stayed just as physical but got a lot faster, no area produced as many great football players or as many distinguished coaches as this region. For the children of the steel mills, whose parents barely spoke English, it was often the first step in the Americanization process, the first recognition as successes they or their families would get, and in some cases, the first ticket out of the steel mills and coal mines, a chance for a college education.
So it was with the Belichicks.