After months of searching high and low for a primer on Applied Probability (I like stuff with Gambling involved cos its immediately applicable!), I did a search for Spread Betting on Google and came across this site: bethelp.com
In their "Learn-to Win" segment I came across several links to Probability as applied to betting:
Mathpages - The Gambler's Ruin
Probabilistic Thinking
Probability Theory
Saliu - The Fund. Formula of Gambling
Saliu - Theory of Probability