If your interested in developing Facebooks apps, these books are a good place to start:
"Facebook became the largest worldwide social site in the middle of last year. If their current pace holds they will pass MySpace as number one in the US some time next year. Those numbers have led a number of people to strike out and develop Facebook applications, hoping to grab a piece of that huge audience. One aspect of writing such applications is knowing Facebook Markup Language, which has been described as the icing on the Facebook API cake. FBML Essentials aims to be the resource that provides hopeful application writers with what they need to use FBML successfully."
You can also get started here with the Facebook developers tutorial