A co-worker lend me this book called "The purpose Driven Life" by Rick Warren. The book is based on Christian teachings, with core point that in order to find your life's purpose, you need to start with God.
IMO, most self help or philosopy books will tell you to search within yourself to find the answer for the Q "Why Am I Here?". As Existentialism stated, we exist first then we find our reason to be here, not the other way around as the above book said. Or maybe the need to reason is very much related when you belief in superior beings, which has power to control your existance (past, present, and future). As Betrand Russel said (which quoted form the book), "Unless you assume a God, the question of life's purpose is meaningless".
What is the purpose of your life? How do you know/find it?