A terrific book that is very helpful as a mom in understanding your kids' birth order roles in your family is The Birth Order Book by Dr. Kevin Leman. I found it to not only be helpful with the kids, but also gaining insight in to my own behaviors and those of others. The book provides fascinating insights into how birth order affects personality, relationships, children, career prospects and parenting styles. It is one of those books that will have you reading a chapter and thinking, "Well, that explains a lot!"
I highly recommend reading it and adding it to your bookshelf as a a useful parenting resource.