I agree about checking out Ebay and looking at completed listings.

You could also get an estimate from an antique book dealer - check Yahoo Yellow Pages for your local area.

Or check out Bookfinder and plug in the title of your book. It will give you an idea what dealers are charging for your titles.

Go to e-bay, search for "baseball books", click on "antiquarian and collectibles", checkmark "completed listings" click on show items button.

This gives a much more accurate and up-to-date market valuation than a price book.