If you really want to get what being prep is all about, you have to be very careful who you read. Birnbach is actually a pretty astute commentator when she wants to be but she is more interested in making money that presenting an honest account of the values. And that is a problem if you are part of the culture.
Preps are self-critical people. They read not to to reinforce their values but to challenge them. This is even true of Birnbach in a not terribly profound way. The preppiest author of them all is Edith Wharton but Wharton is very critical of prep values. Her heroines Lily Bart and Anna Leith are not a role models for prep women but, rather, they are the woman they are afraid of becoming. The same is true of John P Marquand whose The Late George Apley does for men what Lily Bart does for women. (Although it must be said that Marquand is not Wharton's equal or even close to it.)
Another problem is that preps actually read stuff. I mean they pick books up and read them from the start to the end and they go back and read them again and again. That means that just listing a bunch of preppy texts is not much use because most people have images that go with these books that often bear little resemblance to what you actually find in the books if you read them carefully. One of the classics of prep literature, for example, is Walden. Now that may give you a certain impression if you've only ever skimmed the book and heard some prof lecture about it but go back and read it carefully. I always suggest people start by paying attention to Thoreau's attitudes about trains. At first he is bitingly critical of the train but read it through and watch him change his mind and, this is really important, see why he changes his mind. It's a very preppy book.
The final problem is that an awful lot of what you can read about preppies is written by people who hate preps. You have self-loathing preps such as JD Salinger and John Cheever and you have mainstream journalists and university professors who need to attach personalities to the hated "rich"and generally settle on preppy as one of these. The truth is that preps are far less wealthy than they appear. There is often money there to be sure but it's slowly accumulated money and, even then, there is less security than their appears to be. There is a lot of downward mobility in the upper middle class and those of us who are in it are painfully aware of the downside.
So where do you go to get a good portrait of what preps are really like. Whit Stillman is your guy. His movies and his one novel are the best place to start.
Not unrelated, the following clip from his Barcelona is a brilliant explanation of where anti-Americanism comes from. If you replace "Americans" with "Preps" it makes just as good a point.