It's just high school maths, nothing too advanced, but any book about probability should go over the basics. Permutations and combinations using factorials and binomial coefficients (which are the rows of Pascal's triangle) is basic combinatorics stuff, which should be covered in high school maths too, although it's a long time since I was at school ;)
EDIT: Rolling small numbers of dice - it isn't too hard to enumerate all the possibilities or draw a diagram (like AngleWyrm did); rolling 2 dice results in a 2D array and 3 dice results in a 3D lattice of possible outcomes. In my reply about when the order is unimportant I basically enumerated all the possibilities and added up the probabilities of them all to get 1 (i.e. every possibility covered, since the events were disjoint) in order to check that my reasoning was correct.
