Anne,
Read Alfred Adler's What Life Could Mean to You,Chapter 6, "Family Influences, for a good overview. Also look at Lydia Sicher: An Adlerian Perspective, edited by Adele Davidson, Chapter 17, "Family Constellation."
For a contemporary view of birth order influence, check out Robert Boynton's article, "The Birth of an Idea," in the latest issue of The New Yorker magazine. He discusses Frank Sulloway's research and new book, Born to Rebel.