Does anyone know of any good websites where I can find out how much certain old books are worth?

I’ve been digging through my collection of old books, and I’ve found some interesting material, including classified military books from WWII. Is there a good website you know of which can be used to determine the value of old books such as this? Is there any other way of determining the value of these books? Any particular person or business type I should go to? Any help would be appreciated.