Is there any way, on the internet, that I can figure out how much my antique books are worth?

Answer 1

A site like Amazon lists a lot of books, also antiques, but often the prices are lower than what you would pay when you go to a store.
The condition of the books does make a difference, if they are in less than perfect condition, half any price you see on the web, if they are in readable but poor condition, you will only get money for them if they are really rare, very unlikely.

And if you ever sell, a store-owner often will give you 10% of what he hopes to sell for, sometimes even less.

If you need the value of your books for insurance reasons, first look them up on the web, then take (some) to an antique book dealer, telling him that you need to know the condition of the books, so you can insure it properly, and the resale value as second.

Answer 2

I went to E-bay and looked at what other people were selling the same book for. Then ask one sellers the same question.