article from The New York Times notes:
It is, tested by time, one of the most cherished of American novels, recording in its powerful fashion the first years of this century in a breeding place of American genius, Brooklyn's Williamsburg and Greenpoint. In the novel's period these neighborhoods were mostly populated by a poverty-level mix of the two great waves of immigrants, the Irish and the Germans of the mid-19th century and the East European Jews and Italians who followed. . . . The book is a social document with the power of Jacob Riis's photographs. It gives the detail that illuminates the past -- the coffee pot, the air shaft, the barber's cup, chalking strangers on Halloween.
While I enjoyed those passages, what I really liked about A Tree Grows in Brooklyn was that Francie likes books and reading--she likes to escape into their worlds, she likes holding the books in her hands, reading the same books over and over again (If I Were King or Beverly of Graustark); she wants an education and to learn all the time--and she wants to be a writer.