You probably don't have to get a whole book on the subject, if you have an Iphone there are apps out there that will do those calculations for you, plus all quilt stores have litle cheat sheet cards and stuff you can get and use. They are even on the internet, just print them out.
With regard to the length X width = area.....then what?
Say a 49 X 56 quilt top = 2,744, what do you do with that 2,744 number to get it into yards or something figurable?