Both of those books are great, but ISSFLL is Girard at his most brilliant, explaining the role mimetic desire plays in human conflict.

This book is very short, up and to the point. Provided you don't stop at the symbolic language that is only clarified a bit late into the book.

You should start with Things Hidden Since the Foundation of the World. It's a great summary of Girard's theory, and it's easy to read.