basic probability

I just read TOP and HOH, I've always been a gut player, but I do well at mental math, and need to expand the mathematical sector of my game. I need to start at the basics, where do I go for that? Things like converting percentages to odds, and the second and third level probabilities (adding in other possibilities, mathematically considering implied and expected odds).

could we get a sticky or faq? is there a good book for this?

thanks for the help, I look forward to this incredibly improving my game.