I do not know of any online valuers. I am sure they are out there. It is not silly to ask someone about your books - that is what the people who deal in old/valuable/rare books do for their livelihood. If you have a used book store or antique shops, try there. It is true that not all old books are valuable. They are just old. But then you run across one that makes up for the ones that are not.
Have you looked at the Antiques Roadshow's website?
Antiques Roadshow Online | PBS
I had the pleasure of having an old book of mine appraised when Antiques Roadshow came to where I live. It was a book written by Teddy Roosevelt about his ranching experiences in North Dakota. It is not yet "rare". lol.
Have fun and good luck.