I once had this book, and I thought at the time that it was a fairly good primer on existentialism as understood by the makers of the show. It was called existential joss whedon, and it was a nice aid for interested fans of the show not well educated in existentialism to learn about it. I mention it because I think it explored the psychology of the characters from the pov of existentialism too. It has been a long time since I read the book, so I apologize that I can't offer more details about it. you should check it out:
Here it is!