This is going to show my lack of knowledge right off the bat, but could someone explain, or point me to a good book about how to deal with changing exposure times when changing filters with VC paper?

I seem to remember reading about a chart possibly that you could use after you've determined the proper base exposer and then wanted to change the contrast with filters. Do different filters have a factor that you add or subtract to maintain the proper exposure?

Would anyone recommend the Kodak darkroom dataguide? If so, anyone know a good place to look for one at a decent price?