This book is an excellent resource, although because of the number of references, it cannot go into depth on many. But if you are looking for a general understanding of a concept, person, or phrase ingrained in American culture, you will find it here.
I fear that many critics of this book chastize it for its failure to include persons or events near and dear to their hearts. While I am sympathetic to that concern, the reader must understand that this book is akin to a popularity contest of culture, with the most commonly used/understood concepts rising to the top. This is actually a good thing, although it seems shallow at first blush.
As the authors note, the ability to communicate/read well stems from shared understanding. This book succeeds by providing what, at a minimum, should be known by someone because most literate Americans also know it. The authors, in fact, do not suggest we educate ourselves only within the confines of this book, or take its ideas as intrinsically more valuable. Rather, they say only that this is where we must start.
If my friends from abroad asked me what single best reference would prepare them to interact intelligently in America, this would be it.