Questions On Cover Values

I've got a question or two about cover values. I have some WWII-era covers I'm thinking of selling or putting up for action; besides the stamps on a cover, what else goes into assigning a value to it and how would it be determined? For instance, some of these covers are censored or have cachets/special postmarks; I'm sure these add to their value, but don't know how much. Are there such things as catalogs of censored or cacheted covers?

You might want to check the Scott's Specialized Catalog for cover values. I am sure someone has written a book on WWII postal history and patriotic covers. I hope this helps.