Mike Petriv's Hold Em Odds book does a good job of teaching the probability calculations you need for poker, approaching things logically, and in multiple ways if applicable.
Start with that. If it goes too fast, check out the Society of Actuaries web site. I believe its
www.soa.org, but im not positive, do a search if not. There should be a tab for the texts they use for their examinations, and whatever they use for proability will be good. You can probably order it directly from them as well.